WPS: Convert assert() to error return

src/wps/wps_upnp_event.c

@@ -238,9 +238,12 @@ static int event_send_start(struct subscription *s)
 	 * Assume we are called ONLY with no current event and ONLY with
 	 * nonempty event queue and ONLY with at least one address to send to.
 	 */
-	assert(!dl_list_empty(&s->addr_list));
-	assert(s->current_event == NULL);
-	assert(!dl_list_empty(&s->event_queue));
+	if (dl_list_empty(&s->addr_list))
+		return -1;
+	if (s->current_event)
+		return -1;
+	if (dl_list_empty(&s->event_queue))
+		return -1;
 
 	s->current_event = e = event_dequeue(s);