[mutter] display: Fix crash with cycle_group

[Jasper St. Pierre <jstpierre src gnome org>]

commit a3ea43da523eb11c8b39ce329b28475fe99e4394
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Thu Mar 24 02:04:23 2011 -0400

    display: Fix crash with cycle_group
    
    Commit e3ebcc004d6ddb13174cfdd934cb086fb2e50808 introduced an unsafe
    access for 'window', which could cause a segfault.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=645843

 src/core/display.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/core/display.c b/src/core/display.c
index 0514f50..8b1b5f2 100644
--- a/src/core/display.c
+++ b/src/core/display.c
@@ -3528,7 +3528,7 @@ meta_display_begin_grab_op (MetaDisplay *display,
    * grab the parent instead for moving.
    */
   if (meta_prefs_get_attach_modal_dialogs () &&
-      window->type == META_WINDOW_MODAL_DIALOG &&
+      window && window->type == META_WINDOW_MODAL_DIALOG &&
       meta_grab_op_is_moving (op))
     grab_window = meta_window_get_transient_for (window);