tevent: don't skip a fd event if the previous one was deleted during poll()

tevent: don't skip a fd event if the previous one was deleted during poll()

In a threaded environment it can happen that an tevent_fd is talloc_free'ed
while the main thread sleeps in the poll() syscall.

In such a case poll_event_fd_destructor() would set poll_ev->fdes[i] = NULL.

We then skip the removed event, but before we also skipped the one
that was located at the end of the array. We moved it to possition
'i', but the next loop uses 'i=i+1'.

Signed-off-by: Stefan Metzmacher <metze@samba.org>