notifier/listener: fix change notifiers notifying on listen

Change notifiers should only trigger when
notifier_notify is called, not on listen.

Author: ardera

src/notifier_listener.c

@@ -112,10 +112,12 @@ struct listener *notifier_listen(struct notifier *notifier, listener_cb_t notify
         return NULL;
     }
 
-    r = listener_notify(l, notifier->state);
-    if (r == kUnlisten) {
-        listener_destroy(l);
-        return NULL;
+    if (notifier->is_value_notifier) {
+        r = listener_notify(l, notifier->state);
+        if (r == kUnlisten) {
+            listener_destroy(l);
+            return NULL;
+        }
     }
 
     notifier_lock(notifier);