message: fix swServer_start_check bug
  when start the http server, it throw error:
  ```
  WARNING swServer_start_check: onPacket event callback must be set.
  PHP Fatal error:  Swoole\Server::start(): failed to start server. Error: swServer_start_check: onPacket event callback must be set.
  ```
diff: @@ -218,7 +218,7 @@ static int swServer_start_check(swServer *serv)
  return SW_ERR;
  }
  //dgram
  - if (!serv->have_dgram_sock && serv->onPacket == NULL)
  + if (serv->have_dgram_sock && serv->onPacket == NULL)
  {
  swWarn("onPacket event callback must be set.");
  return SW_ERR;
  