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.

library/ssl_cli.c

@@ -2173,7 +2173,7 @@ static int ssl_parse_server_key_exchange( ssl_context *ssl )
              * };
              */
             if( ( ret = md_init_ctx( &ctx,
-                                     md_info_from_type( md_alg ) ) ) != 0 )
+                                     md_info_from_type( md_alg ), 0 ) ) != 0 )
             {
                 SSL_DEBUG_RET( 1, "md_init_ctx", ret );
                 return( ret );