Fix bug #5814 - Winbindd dumping core in a strange manner while doing "rescan_trusted...

Fix bug #5814 - Winbindd dumping core in a strange manner while doing "rescan_trusted_domain".
From analysis by hargagan <shargagan@novell.com> :
"The winbindd_child_died() is also getting called from process_loop() in case of
SIGCHLD signal. In this case it doesn't make the timeout_handler to NULL for
the first request. It then initiate a new request using
schedule_async_request() which installs a new timeout handler for the same
request. In such a case, for a badly unresponsive system both the timeout
handler can be called. For the first call the "private_data" will be cleared
and for another call the timeout handler will be detecting the double free. So,
for such a case as well, the winbindd_child_died() should make the
timeout_handler to NULL."
Jeremy.
(cherry picked from commit ce8de496ec139b7a56db20c5ffbcbdc2f4db0a51)