[mutter] compositor: Fix crash in pre_paint_windows when no windows are open

[Adel Gadllah <agadllah src gnome org>]

commit 2f33d85a419df9755611113483e9e4431e583978
Author: Adel Gadllah <adel gadllah gmail com>
Date:   Tue Aug 30 12:42:01 2011 +0200

    compositor: Fix crash in pre_paint_windows when no windows are open
    
    https://bugzilla.gnome.org/show_bug.cgi?id=657692

 src/compositor/compositor.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index 9c85c7d..ccda344 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -1145,6 +1145,9 @@ pre_paint_windows (MetaCompScreen *info)
   MetaWindowActor *top_window;
   MetaWindowActor *expected_unredirected_window = NULL;
 
+  if (info->windows == NULL)
+    return;
+
   top_window = g_list_last (info->windows)->data;
 
   if (meta_window_actor_should_unredirect (top_window) &&