Fix a crash that can result if text codecs are allowed but textsupport is disabled.

(closes issue #15596)
Reported by: fabled
Patches:
      sip-red.patch uploaded by fabled (license 448)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@209516 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -8611,7 +8611,7 @@ static int process_sdp(struct sip_pvt *p, struct sip_request *req, int t38action
 		p->jointcapability = ast_codec_choose(&p->prefs, p->jointcapability, 1);
 	}
 
-	if (p->jointcapability & AST_FORMAT_T140RED) {
+	if (p->trtp && (p->jointcapability & AST_FORMAT_T140RED)) {
 		p->red = 1;
 		ast_rtp_red_init(p->trtp, 300, red_data_pt, 2);
 	} else {