Fix a linked list corruption that under the right circumstances

could lead to a looped list, meaning it will traverse forever.

(closes issue #11818)
Reported by: michael-fig
Patches:
      11818.patch uploaded by putnopvut (license 60)
	  Tested by: michael-fig



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@103770 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_zap.c

@@ -7706,7 +7706,10 @@ static struct zt_pvt *chandup(struct zt_pvt *src)
 	}
 	p->destroy = 1;
 	p->next = iflist;
+	p->prev = NULL;
 	iflist = p;
+	if (iflist->next)
+		iflist->next->prev = p;
 	return p;
 }