防止短期获取同一个poller对象,当所有线程的负载都是0时，min_load == 0会直接breadk,导致thread_pos不会加1

[scopys]

在WorkThreadPool::Instance()->getPoller()的线程中又再次调用WorkThreadPool::Instance()->getPoller()，而计算负载的可能还是0，从而获取到了同一个poller

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