fix seg in recent change

libs/freetdm/mod_freetdm/mod_freetdm.c

@@ -545,7 +545,7 @@ static switch_status_t channel_on_hangup(switch_core_session_t *session)
 	tokencnt = ftdm_channel_get_token_count(tech_pvt->ftdmchan);
 	for (t = 0; t < tokencnt; t++) {
 		token = ftdm_channel_get_token(tech_pvt->ftdmchan, tokencnt);
-		if (!strcasecmp(uuid, token)) {
+		if (!zstr(token) && !strcasecmp(uuid, token)) {
 			uuid_found = 1;
 			break;
 		}