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
........

Merged revisions 434449 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434450 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_queue.c

@@ -3941,6 +3941,7 @@ static void leave_queue(struct queue_ent *qe)
 			while ((pr_iter = AST_LIST_REMOVE_HEAD(&qe->qe_rules, list))) {
 				ast_free(pr_iter);
 			}
+			qe->pr = NULL;
 			snprintf(posstr, sizeof(posstr), "%d", qe->pos);
 			pbx_builtin_setvar_helper(qe->chan, "QUEUEPOSITION", posstr);
 		} else {