app_queue crashes randomly, often during call-transfers

This patch adds a ref to the queue_ent object's parent call_queue
in queue_exec() so the call_queue won't be destroyed
while the the queue_ent still holds a pointer to it.

(closes issue 0015686)
Tested by: dvossel, aragon




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@231556 65c4cc65-6c06-0410-ace0-fbb531ad65f3