[metacity] Fix very unlikely crasher. See https://bugzilla.gnome.org/attachment.cgi?id=133411

[Thomas James Alexander Thurman <tthurman src gnome org>]

commit 5ddd8199ca1fe05f066bf05b35cd472bb6ff4703
Author: Paul Liu <grandpaul gmail com>
Date:   Sat May 8 17:39:28 2010 -0400

    Fix very unlikely crasher.  See https://bugzilla.gnome.org/attachment.cgi?id=133411

 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 f5cbdce..429230d 100644
--- a/src/core/display.c
+++ b/src/core/display.c
@@ -4319,7 +4319,7 @@ find_tab_forward (MetaDisplay   *display,
     }
 
   tmp = workspace->mru_list;
-  while (tmp != start)
+  while (tmp != start && tmp != NULL)
     {
       MetaWindow *window = tmp->data;