message: fix seria string core dump
diff: @@ -125,7 +125,7 @@ int php_swoole_task_pack(swEventData *task, zval *data TSRMLS_DC)
  }
  #if PHP_MAJOR_VERSION >= 7
  - if (SWOOLE_G(fast_serialize))
  + if (SWOOLE_G(fast_serialize) && serialized_string)
  {
  zend_string_release(serialized_string);
  }
  