apps/app_queue: Prevent possible crash when evaluating queue penalty rules

apps/app_queue: Prevent possible crash when evaluating queue penalty rules

Although it only occurred once, a crash occurred when a queue attempted to
evaluate a queue penalty rule that appeared to have already been destroyed.
In many locations in app_queue, a test is done to see if qe->pr is NULL;
however, when we dispose of a queue's penalty rules, we don't set the pointer
to NULL after free'ing it. This patch does that to prevent any dangling
pointers from lingering on the queue object.

Review: https://reviewboard.asterisk.org/r/4522

ASTERISK-23319 #close
Reported by: Vadim
patches:
  rb4552.patch submitted by Stefan Engström (License 6691)
........

Merged revisions 434448 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434449 65c4cc65-6c06-0410-ace0-fbb531ad65f3