Make sure the pvt exists before accessing it again as it may have gone away (issue #7562 reported by Seb7 and issue #7939 reported by sorg)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@43915 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_iax2.c

@@ -7579,7 +7579,7 @@ retryowner2:
 	if (duped_fr) {
 		schedule_delivery(duped_fr, updatehistory, 0, &fr->ts);
 	}
-	if (iaxs[fr->callno]->last < fr->ts) {
+	if (iaxs[fr->callno] && iaxs[fr->callno]->last < fr->ts) {
 		iaxs[fr->callno]->last = fr->ts;
 #if 1
 		if (option_debug && iaxdebug)