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/programs/aes/crypt_and_hash.c b/programs/aes/crypt_and_hash.c
index a1406b6..2a3f3cc 100644
--- a/programs/aes/crypt_and_hash.c
+++ b/programs/aes/crypt_and_hash.c
@@ -185,7 +185,7 @@
         polarssl_fprintf( stderr, "Message Digest '%s' not found\n", argv[5] );
         goto exit;
     }
-    md_init_ctx( &md_ctx, md_info);
+    md_init_ctx( &md_ctx, md_info, 1 );
 
     /*
      * Read the secret key and clean the command line.