Make vlc_object_detach a no-op if the object is not attached

src/misc/objects.c

@@ -918,7 +918,10 @@ void __vlc_object_attach( vlc_object_t *p_this, vlc_object_t *p_parent )
 
 static void vlc_object_detach_unlocked (vlc_object_t *p_this)
 {
-    assert (p_this->p_parent);
+    vlc_assert_locked (&structure_lock);
+
+    if (p_this->p_parent == NULL)
+        return;
 
     vlc_object_internals_t *priv = vlc_internals( p_this->p_parent );