mysql: don't crash when --show-warnings is enabled

and the server suddenly goes away

client/mysql.cc

@@ -3588,7 +3588,7 @@ static void print_warnings()
   mysql_store_result_for_lazy(&result);
 
   /* Bail out when no warnings */
-  if (!(num_rows= mysql_num_rows(result)))
+  if (!result || !(num_rows= mysql_num_rows(result)))
     goto end;
 
   cur= mysql_fetch_row(result);