Zeroize sensitive data in aescrypt2 and crypt_and_hash examples

This commit replaces multiple `memset()` calls in the example
programs aes/aescrypt2.c and aes/crypt_and_hash.c by calls to
the reliable zeroization function `mbedtls_zeroize()`.

While not a security issue because the code is in the example
programs, it's bad practice and should be fixed.
diff --git a/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index bc95eb9..88b852b 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -46,6 +46,7 @@
  defined(MBEDTLS_FS_IO)
 #include "mbedtls/cipher.h"
 #include "mbedtls/md.h"
+#include "mbedtls/platform_util.h"
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -547,13 +548,13 @@
        the case when the user has missed or reordered some,
        in which case the key might not be in argv[6]. */
     for( i = 0; i < argc; i++ )
-        memset( argv[i], 0, strlen( argv[i] ) );
+        mbedtls_platform_zeroize( argv[i], strlen( argv[i] ) );
 
-    memset( IV,     0, sizeof( IV ) );
-    memset( key,    0, sizeof( key ) );
-    memset( buffer, 0, sizeof( buffer ) );
-    memset( output, 0, sizeof( output ) );
-    memset( digest, 0, sizeof( digest ) );
+    mbedtls_platform_zeroize( IV,     sizeof( IV ) );
+    mbedtls_platform_zeroize( key,    sizeof( key ) );
+    mbedtls_platform_zeroize( buffer, sizeof( buffer ) );
+    mbedtls_platform_zeroize( output, sizeof( output ) );
+    mbedtls_platform_zeroize( digest, sizeof( digest ) );
 
     mbedtls_cipher_free( &cipher_ctx );
     mbedtls_md_free( &md_ctx );