freetdm: ss7 -bug fix

libs/freetdm/src/ftmod/ftmod_sangoma_ss7/ftmod_sangoma_ss7_main.c

@@ -1240,6 +1240,7 @@ static ftdm_status_t ftdm_sangoma_ss7_start(ftdm_span_t * span)
 	for (x = 1; x < (span->chan_count + 1); x++) {
 		/* extract the channel structure and sngss7 channel data */
 		ftdmchan = span->channels[x];
+		if (ftdmchan->call_data == NULL) continue;
 		sngss7_info = ftdmchan->call_data;
 		sngss7_span = ftdmchan->span->mod_data;