Bug 224427 - Get this when visiting the site: "Gtk-WARNING **: invalid cast from 'GtkSuperWin' to 'GtkWidget'"

Patch contributed by Pavel Kankovsky <peak@argo.troja.mff.cuni.cz>
r=caillon@gmail.com, sr=blizzard@mozilla.org

modules/plugin/base/src/ns4xPluginInstance.cpp

@@ -1095,7 +1095,7 @@ NS_IMETHODIMP ns4xPluginInstance::SetWindow(nsPluginWindow* window)
 
   gpointer user_data = nsnull;
   gdk_window_get_user_data(win, &user_data);
-  if (user_data) {
+  if (user_data && GTK_IS_WIDGET(user_data)) {
     GtkWidget* widget = GTK_WIDGET(user_data);
 
     if (GTK_IS_SOCKET(widget))