Introduce MD handle type

As has been previously done for ciphersuites, this commit introduces
a zero-cost abstraction layer around the type

  mbedtls_md_info const *

whose valid values represent implementations of message digest algorithms.

Access to a particular digest implementation can be requested by name or
digest ID through the API mbedtls_md_info_from_xxx(), which either returns
a valid implementation or NULL, representing failure.

This commit replaces such uses of `mbedtls_md_info const *` by an abstract
type `mbedtls_md_handle_t` whose valid values represent digest implementations,
and which has a designated invalid value MBEDTLS_MD_INVALID_HANDLE.

The purpose of this abstraction layer is to pave the way for builds which
support precisely one digest algorithm. In this case, mbedtls_md_handle_t
can be implemented as a two-valued type, with one value representing the
invalid handle, and the unique valid value representing the unique enabled
digest.
diff --git a/tests/suites/test_suite_md.function b/tests/suites/test_suite_md.function
index 11cf88a..515a28c 100644
--- a/tests/suites/test_suite_md.function
+++ b/tests/suites/test_suite_md.function
@@ -11,7 +11,7 @@
 void mbedtls_md_process(  )
 {
     const int *md_type_ptr;
-    const mbedtls_md_info_t *info;
+    mbedtls_md_handle_t info;
     mbedtls_md_context_t ctx;
     unsigned char buf[150];
 
@@ -28,7 +28,7 @@
     for( md_type_ptr = mbedtls_md_list(); *md_type_ptr != 0; md_type_ptr++ )
     {
         info = mbedtls_md_info_from_type( *md_type_ptr );
-        TEST_ASSERT( info != NULL );
+        TEST_ASSERT( info != MBEDTLS_MD_INVALID_HANDLE );
         TEST_ASSERT( mbedtls_md_setup( &ctx, info, 0 ) == 0 );
         TEST_ASSERT( mbedtls_md_process( &ctx, buf ) == 0 );
         mbedtls_md_free( &ctx );
@@ -43,18 +43,22 @@
 void md_null_args(  )
 {
     mbedtls_md_context_t ctx;
-    const mbedtls_md_info_t *info = mbedtls_md_info_from_type( *( mbedtls_md_list() ) );
+    mbedtls_md_handle_t info = mbedtls_md_info_from_type( *( mbedtls_md_list() ) );
     unsigned char buf[1] = { 0 };
 
     mbedtls_md_init( &ctx );
 
-    TEST_ASSERT( mbedtls_md_get_size( NULL ) == 0 );
-    TEST_ASSERT( mbedtls_md_get_type( NULL ) == MBEDTLS_MD_NONE );
-    TEST_ASSERT( mbedtls_md_get_name( NULL ) == NULL );
+    TEST_ASSERT( mbedtls_md_get_size( MBEDTLS_MD_INVALID_HANDLE )
+                 == 0 );
+    TEST_ASSERT( mbedtls_md_get_type( MBEDTLS_MD_INVALID_HANDLE )
+                 == MBEDTLS_MD_NONE );
+    TEST_ASSERT( mbedtls_md_get_name( MBEDTLS_MD_INVALID_HANDLE )
+                 == NULL );
 
-    TEST_ASSERT( mbedtls_md_info_from_string( NULL ) == NULL );
+    TEST_ASSERT( mbedtls_md_info_from_string( NULL ) == MBEDTLS_MD_INVALID_HANDLE );
 
-    TEST_ASSERT( mbedtls_md_setup( &ctx, NULL, 0 ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
+    TEST_ASSERT( mbedtls_md_setup( &ctx, MBEDTLS_MD_INVALID_HANDLE, 0 )
+                 == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
     TEST_ASSERT( mbedtls_md_setup( NULL, info, 0 ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
     TEST_ASSERT( mbedtls_md_starts( NULL ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
@@ -66,10 +70,12 @@
     TEST_ASSERT( mbedtls_md_finish( NULL, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
     TEST_ASSERT( mbedtls_md_finish( &ctx, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
-    TEST_ASSERT( mbedtls_md( NULL, buf, 1, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
+    TEST_ASSERT( mbedtls_md( MBEDTLS_MD_INVALID_HANDLE,
+                             buf, 1, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
 #if defined(MBEDTLS_FS_IO)
-    TEST_ASSERT( mbedtls_md_file( NULL, "", buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
+    TEST_ASSERT( mbedtls_md_file( MBEDTLS_MD_INVALID_HANDLE,
+                                  "", buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 #endif
 
     TEST_ASSERT( mbedtls_md_hmac_starts( NULL, buf, 1 )
@@ -90,27 +96,29 @@
     TEST_ASSERT( mbedtls_md_hmac_reset( NULL ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
     TEST_ASSERT( mbedtls_md_hmac_reset( &ctx ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
-    TEST_ASSERT( mbedtls_md_hmac( NULL, buf, 1, buf, 1, buf )
+    TEST_ASSERT( mbedtls_md_hmac( MBEDTLS_MD_INVALID_HANDLE, buf, 1, buf, 1, buf )
                  == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
     TEST_ASSERT( mbedtls_md_process( NULL, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
     TEST_ASSERT( mbedtls_md_process( &ctx, buf ) == MBEDTLS_ERR_MD_BAD_INPUT_DATA );
 
     /* Ok, this is not NULL arg but NULL return... */
-    TEST_ASSERT( mbedtls_md_info_from_type( MBEDTLS_MD_NONE ) == NULL );
-    TEST_ASSERT( mbedtls_md_info_from_string( "no such md" ) == NULL );
+    TEST_ASSERT( mbedtls_md_info_from_type( MBEDTLS_MD_NONE ) ==
+                 MBEDTLS_MD_INVALID_HANDLE );
+    TEST_ASSERT( mbedtls_md_info_from_string( "no such md" ) ==
+                 MBEDTLS_MD_INVALID_HANDLE );
 }
 /* END_CASE */
 
 /* BEGIN_CASE */
 void md_info( int md_type, char * md_name, int md_size )
 {
-    const mbedtls_md_info_t *md_info;
+    mbedtls_md_handle_t md_info;
     const int *md_type_ptr;
     int found;
 
     md_info = mbedtls_md_info_from_type( md_type );
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
     TEST_ASSERT( md_info == mbedtls_md_info_from_string( md_name ) );
 
     TEST_ASSERT( mbedtls_md_get_type( md_info ) == (mbedtls_md_type_t) md_type );
@@ -132,7 +140,7 @@
     char md_name[100];
     unsigned char src_str[1000];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
 
     memset( md_name, 0x00, 100 );
     memset( src_str, 0x00, 1000 );
@@ -141,7 +149,7 @@
     strncpy( (char *) src_str, text_src_string, sizeof( src_str ) - 1 );
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string(md_name);
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
 
     TEST_ASSERT ( 0 == mbedtls_md( md_info, src_str, strlen( (char *) src_str ), output ) );
 
@@ -155,14 +163,14 @@
 {
     char md_name[100];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
 
     memset( md_name, 0x00, 100 );
     memset( output, 0x00, 100 );
 
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string( md_name );
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
 
     TEST_ASSERT ( 0 == mbedtls_md( md_info, src_str->x, src_str->len, output ) );
 
@@ -181,7 +189,7 @@
     unsigned char output[100];
     int halfway, len;
 
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
     mbedtls_md_context_t ctx, ctx_copy;
 
     mbedtls_md_init( &ctx );
@@ -197,7 +205,7 @@
     halfway = len / 2;
 
     md_info = mbedtls_md_info_from_string(md_name);
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
     TEST_ASSERT ( 0 == mbedtls_md_setup( &ctx, md_info, 0 ) );
     TEST_ASSERT ( 0 == mbedtls_md_setup( &ctx_copy, md_info, 0 ) );
 
@@ -230,7 +238,7 @@
 {
     char md_name[100];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
     mbedtls_md_context_t ctx, ctx_copy;
     int halfway;
 
@@ -242,7 +250,7 @@
 
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string(md_name);
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
     TEST_ASSERT ( 0 == mbedtls_md_setup( &ctx, md_info, 0 ) );
     TEST_ASSERT ( 0 == mbedtls_md_setup( &ctx_copy, md_info, 0 ) );
 
@@ -277,14 +285,14 @@
 {
     char md_name[100];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
 
     memset( md_name, 0x00, 100 );
     memset( output, 0x00, 100 );
 
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string( md_name );
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
 
 
     TEST_ASSERT ( mbedtls_md_hmac( md_info, key_str->x, key_str->len, src_str->x, src_str->len, output ) == 0 );
@@ -299,7 +307,7 @@
 {
     char md_name[100];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
     mbedtls_md_context_t ctx;
     int halfway;
 
@@ -310,7 +318,7 @@
 
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string( md_name );
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
     TEST_ASSERT ( 0 == mbedtls_md_setup( &ctx, md_info, 1 ) );
 
     halfway = src_str->len / 2;
@@ -344,14 +352,14 @@
 {
     char md_name[100];
     unsigned char output[100];
-    const mbedtls_md_info_t *md_info = NULL;
+    mbedtls_md_handle_t md_info = MBEDTLS_MD_INVALID_HANDLE;
 
     memset( md_name, 0x00, 100 );
     memset( output, 0x00, 100 );
 
     strncpy( (char *) md_name, text_md_name, sizeof( md_name ) - 1 );
     md_info = mbedtls_md_info_from_string( md_name );
-    TEST_ASSERT( md_info != NULL );
+    TEST_ASSERT( md_info != MBEDTLS_MD_INVALID_HANDLE );
 
     TEST_ASSERT( mbedtls_md_file( md_info, filename, output ) == 0 );