Do not try to unlock a non-existant channel if the transfer fails.

(closes issue #13800)
Reported by: dwagner
Patches:
      asterisk-1.4.22-chan-sip-nullp.patch uploaded by tweety (license 608)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@164350 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -14313,7 +14313,9 @@ static int handle_request_invite(struct sip_pvt *p, struct sip_request *req, int
 			ast_mutex_unlock(&p->lock);
 			if (p->refer->refer_call) {
 				ast_mutex_unlock(&p->refer->refer_call->lock);
-				ast_channel_unlock(p->refer->refer_call->owner);
+				if (p->refer->refer_call->owner) {
+					ast_channel_unlock(p->refer->refer_call->owner);
+				}
 			}
 			p->invitestate = INV_COMPLETED;
 			return -1;