CONC-622: Fix double free() if asnyc connect failed

If mysql_real_connect_start/cont failed, we need to
set async->pvio to zero after calling ma_pvio_close
to avoid double free() in mysql_close.

unittest/libmariadb/connection.c

@@ -1667,6 +1667,7 @@ static int test_conc312(MYSQL *my)
 
   if (rc)
   {
+    diag("Error: %s", mysql_error(my));
     diag("caching_sha256_password not supported");
     return SKIP; 
   }