tevent: fix crash bug in tevent_queue_immediate_trigger()

tevent: fix crash bug in tevent_queue_immediate_trigger()

Assume we we have a queue with 2 entries (A and B with triggerA() and triggerB()).
If triggerA() removes itself tevent_queue_entry_destructor() will be called
for A, this schedules the immediate event to call triggerB().
If triggerA() then also removes B by an explicit of implizit talloc_free(),
q->list is NULL, but the immediate event is still scheduled and can't be unscheduled.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>