be async: avoid double close()

In case when between this two close (close(F), close(F)) some open()
will be executed, than we will close newly opened fd.

Reported-by: xujiezhige@163.com
Merge pull request #189 from azat/be_async-avoid-doule-close

be async: avoid double close()
Merge branch 'master' of https://github.com/libevent/libevent