res_pjsip_t38: Make t38_reinvite_response_cb tolerant of NULL channel

t38_reinvite_response_cb can get called by res_pjsip_session's
session_inv_on_tsx_state_changed in situations where session->channel
is NULL.  If it is, the ast_log warning segfaults because it tries
to get the channel name from a NULL channel.

* Check session->channel and print "unknown channel" when it's NULL.

ASTERISK-27236
Reported by: Ross Beer

Change-Id: I4326e288d36327f6c79ab52226d54905cdc87dc7

res/res_pjsip_t38.c

@@ -294,21 +294,22 @@ static int t38_reinvite_response_cb(struct ast_sip_session *session, pjsip_rx_da
 {
 	struct pjsip_status_line status = rdata->msg_info.msg->line.status;
 	struct t38_state *state;
-	RAII_VAR(struct ast_sip_session_media *, session_media, NULL, ao2_cleanup);
+	struct ast_sip_session_media *session_media = NULL;
 
 	if (status.code == 100) {
 		return 0;
 	}
 
-	if (!(state = t38_state_get_or_alloc(session)) ||
+	if (!session->channel || !(state = t38_state_get_or_alloc(session)) ||
 		!(session_media = ao2_find(session->media, "image", OBJ_KEY))) {
 		ast_log(LOG_WARNING, "Received response to T.38 re-invite on '%s' but state unavailable\n",
-			ast_channel_name(session->channel));
+			session->channel ? ast_channel_name(session->channel) : "unknown channel");
 		return 0;
 	}
 
 	t38_change_state(session, session_media, state, (status.code == 200) ? T38_ENABLED : T38_REJECTED);
 
+	ao2_cleanup(session_media);
 	return 0;
 }