In function nni_posix_poll_thr losing EPOLLHUP flag causes the code in this function tcp_cb to be executed incorrectly and continue

[wangchen61698]

Describe the bug
I encountered potential bugs using nng（v1.5.2）
In posix_pollq_epoll.c,fuction nni_posix_poll_thr,the value mask will recoder EPOLLIN,EPOLLOUT and EPOLLERR these flag(line279 ~ line281).If the ev->events includes EPOLLHUP flag，The mask will lose EPOLLHUP flag.

When the excuted cb is function tcp_cb
The check condition of the event(posix_tcpconn.c line236 ~239) is to check NNI_POLL_HUP,NNI_POLL_ERR and NNI_POLL_INVAL.

However in fuction nni_posix_poll_thr, It loses EPOLLHUP flag,the tcp_cb don’t check the flag,The code continues to execute incorrectly