the code of pullMessageAsync will lead to crash

[jonnxu]

void DefaultMQPushConsumer::pullMessageAsync(PullRequest* request) {
if (request == NULL || request->isDroped()) {
LOG_WARN("Pull request is set drop with mq:%s, return",(request->m_messageQueue).toString().c_str());
request->removePullMsgEvent();
return;
}
。。。。
}