freetdm: Fix redmine issue #2931 - Crash on SIP to SS7 call after sigbridge call

         The code was improperly using peer_data as an indicator that the sigbridge
         ss7 mode was enabled. The channel flag FTDM_CHANNEL_NATIVE_SIGBRDIGE should
         be used instead

libs/freetdm/src/ftdm_io.c

@@ -2449,6 +2449,12 @@ FT_DECLARE(ftdm_status_t) ftdm_get_channel_from_string(const char *string_id, ft
 	*out_span = NULL;
 	*out_channel = NULL;
 
+	if (!string_id) {
+		ftdm_log(FTDM_LOG_ERROR, "Cannot parse NULL channel id string\n");
+		status = FTDM_EINVAL;
+		goto done;
+	}
+
 	rc = sscanf(string_id, "%u:%u", &span_id, &chan_id);
 	if (rc != 2) {
 		ftdm_log(FTDM_LOG_ERROR, "Failed to parse channel id string '%s'\n", string_id);

libs/freetdm/src/ftmod/ftmod_sangoma_ss7/ftmod_sangoma_ss7_out.c

@@ -57,12 +57,12 @@ void ft_to_sngss7_iam (ftdm_channel_t * ftdmchan)
 	
 	memset (&iam, 0x0, sizeof (iam));
 
-	var = ftdm_usrmsg_get_var(ftdmchan->usrmsg, "sigbridge_peer");
-	if (!ftdm_strlen_zero(var)) {
+	if (ftdm_test_flag(ftdmchan, FTDM_CHANNEL_NATIVE_SIGBRIDGE)) {
 		ftdm_span_t *peer_span = NULL;
 		ftdm_channel_t *peer_chan = NULL;
 		sngss7_chan_data_t *peer_info = NULL;
 
+		var = ftdm_usrmsg_get_var(ftdmchan->usrmsg, "sigbridge_peer");
 		ftdm_get_channel_from_string(var, &peer_span, &peer_chan);
 		if (!peer_chan) {
 			SS7_ERROR_CHAN(ftdmchan, "Failed to find sigbridge peer from string '%s'\n", var);
@@ -91,7 +91,7 @@ void ft_to_sngss7_iam (ftdm_channel_t * ftdmchan)
 		}
 	}
 
-	if (sngss7_info->peer_data) {
+	if (ftdm_test_flag(ftdmchan, FTDM_CHANNEL_NATIVE_SIGBRIDGE) && sngss7_info->peer_data) {
 		sngss7_span_data_t *span_data = ftdmchan->span->signal_data;
 		sngss7_event_data_t *event_clone = ftdm_queue_dequeue(sngss7_info->peer_data->event_queue);
 		/* Retrieve IAM from our peer */