AST-2021-007 - res_pjsip_session: Don't offer if no channel exists.

If a re-INVITE is received after we have sent a BYE request then it
is possible for no channel to be present on the session. If this
occurs we allow PJSIP to produce the offer instead. Since the call
is being hung up if it produces an incorrect offer it doesn't
actually matter. This also ensures that code which produces SDP
does not need to handle if a channel is not present.

ASTERISK-29381

Change-Id: I673cb88c432f38f69b2e0851d55cc57a62236042
(cherry picked from commit 523a795289)

res/res_pjsip_session.c

@@ -5263,6 +5263,16 @@ static void session_inv_on_create_offer(pjsip_inv_session *inv, pjmedia_sdp_sess
 	pjmedia_sdp_session *offer;
 	int i;
 
+	/* We allow PJSIP to produce an SDP if no channel is present. This may result
+	 * in an incorrect SDP occurring, but if no channel is present then we are in
+	 * the midst of a BYE and are hanging up. This ensures that all the code to
+	 * produce an SDP doesn't need to worry about a channel being present or not,
+	 * just in case.
+	 */
+	if (!session->channel) {
+		return;
+	}
+
 	if (inv->neg) {
 		if (pjmedia_sdp_neg_was_answer_remote(inv->neg)) {
 			pjmedia_sdp_neg_get_active_remote(inv->neg, &previous_sdp);