Fix iOS crash issues

Relate to issues #327 and #63.
mosq->sock may be closed before FD_SET(mosq->sock, &writefds) and
FD_ISSET(mosq->sock, &writefds) but after judgement in line 947
if(mosq->sock != INVALID_SOCKET). FD_SET(-1, ...) and FD_ISSET(-1, ...)
would certainly crash.

Signed-off-by: Zard1096 <mr.zardqi@gmail.com>