lib: tevent: Fix bug in poll backend - poll_event_loop_poll()

lib: tevent: Fix bug in poll backend - poll_event_loop_poll()

If the (pfd->revents & POLLNVAL) case is triggered,
we do DLIST_REMOVE(ev->fd_events, fde); and then
use fde->next in the loop above.

Save off fde->next for loop interation before
this so we can't use a deleted ->next value.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11771

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
(cherry picked from commit 2be3dd1407eabe3df360ede2eab178848e34733c)