Don't try to send a response on a NULL pvt.

(closes issue #13919)
 Reported by: barthpbx
 Patches: 
       chan_iax2.c.patch uploaded by eliel (license 64)
 Tested by: barthpbx


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@159269 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_iax2.c

@@ -6271,7 +6271,7 @@ static int update_registry(struct sockaddr_in *sin, int callno, char *devtype, i
 
 	/* Make sure our call still exists, an INVAL at the right point may make it go away */
 	if (!iaxs[callno]) {
-		res = 0;
+		res = -1;
 		goto return_unref;
 	}