PJSIP: avoid crash when getting rtp peer

Although unlikely, if the tech private is returned as
a NULL, chan_pjsip_get_rtp_peer() would crash.

ASTERISK-25323

Change-Id: Ie231369bfa7da926fb2b9fdaac228261a3152e6a

channels/chan_pjsip.c

@@ -160,10 +160,10 @@ static struct ast_sip_session_supplement chan_pjsip_ack_supplement = {
 static enum ast_rtp_glue_result chan_pjsip_get_rtp_peer(struct ast_channel *chan, struct ast_rtp_instance **instance)
 {
 	struct ast_sip_channel_pvt *channel = ast_channel_tech_pvt(chan);
-	struct chan_pjsip_pvt *pvt = channel->pvt;
+	struct chan_pjsip_pvt *pvt;
 	struct ast_sip_endpoint *endpoint;
 
-	if (!pvt || !channel->session || !pvt->media[SIP_MEDIA_AUDIO]->rtp) {
+	if (!channel || !channel->session || !(pvt = channel->pvt) || !pvt->media[SIP_MEDIA_AUDIO]->rtp) {
 		return AST_RTP_GLUE_RESULT_FORBID;
 	}