message: fixed server::finish crash when enable swoole.fast_serialize
diff: @@ -522,7 +522,7 @@ static int php_swoole_task_finish(swServer *serv, zval *data TSRMLS_DC)
  ret = swTaskWorker_finish(serv, data_str, data_len, flags);
  #if PHP_MAJOR_VERSION >= 7
  - if (SWOOLE_G(fast_serialize))
  + if (SWOOLE_G(fast_serialize) && serialized_string)
  {
  zend_string_release(serialized_string);
  }
  