Fix #550 by allowing an empty trigger match list

1e7659609b is related
to this change and required to fix #550 as well.

src/trigger.c

@@ -80,6 +80,8 @@ SR_API void sr_trigger_free(struct sr_trigger *trig)
 
 	for (l = trig->stages; l; l = l->next) {
 		stage = l->data;
+
+		if (stage->matches)
 			g_slist_free_full(stage->matches, g_free);
 	}
 	g_slist_free_full(trig->stages, g_free);