Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/library/rsa.c b/library/rsa.c
index d3ab4ed..afa1c72 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -556,7 +556,7 @@
     memcpy( p, input, ilen );
 
     md_init( &md_ctx );
-    md_init_ctx( &md_ctx, md_info );
+    md_init_ctx( &md_ctx, md_info, 0 );
 
     // maskedDB: Apply dbMask to DB
     //
@@ -725,7 +725,7 @@
     hlen = md_get_size( md_info );
 
     md_init( &md_ctx );
-    md_init_ctx( &md_ctx, md_info );
+    md_init_ctx( &md_ctx, md_info, 0 );
 
     /* Generate lHash */
     md( md_info, label, label_len, lhash );
@@ -969,7 +969,7 @@
     p += slen;
 
     md_init( &md_ctx );
-    md_init_ctx( &md_ctx, md_info );
+    md_init_ctx( &md_ctx, md_info, 0 );
 
     // Generate H = Hash( M' )
     //
@@ -1201,7 +1201,7 @@
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
     md_init( &md_ctx );
-    md_init_ctx( &md_ctx, md_info );
+    md_init_ctx( &md_ctx, md_info, 0 );
 
     mgf_mask( p, siglen - hlen - 1, p + siglen - hlen - 1, hlen, &md_ctx );