Revert "Prevent crash when thumbnailer is not defined (#385)"

This reverts commit 34d802de.

thunar/thunar-thumbnailer.c

@@ -329,8 +329,7 @@ thunar_thumbnailer_queue_async_reply (GObject      *proxy,
 
   thumbnailer = THUNAR_THUMBNAILER (job->thumbnailer);
 
-  if (THUNAR_IS_THUMBNAILER (thumbnailer) == FALSE)
-    return;
+  _thunar_return_if_fail (THUNAR_IS_THUMBNAILER (thumbnailer));
 
   _thumbnailer_lock (thumbnailer);