Protect against X errors when clearing the DND cache

This was reported as a problem in bug 609952.

gdk/x11/gdkdnd-x11.c

@@ -580,8 +580,14 @@ gdk_window_cache_destroy (GdkWindowCache *cache)
   gdk_window_remove_filter (root_window, gdk_window_cache_filter, cache);
   gdk_window_remove_filter (NULL, gdk_window_cache_shape_filter, cache);
 
+  gdk_error_trap_push ();
+
   g_list_foreach (cache->children, (GFunc)free_cache_child,
       gdk_screen_get_display (cache->screen));
+
+  gdk_flush ();
+  gdk_error_trap_pop ();
+
   g_list_free (cache->children);
   g_hash_table_destroy (cache->child_hash);