[mutter] Fix crash in meta_display_get_above_tab_keycode

[Owen Taylor <otaylor src gnome org>]

commit 05662b678e2c19ebc9e1aff63759a5878c2c2f31
Author: Adam Jackson <ajax redhat com>
Date:   Thu Apr 14 10:10:40 2011 -0400

    Fix crash in meta_display_get_above_tab_keycode
    
    https://bugzilla.gnome.org/show_bug.cgi?id=647777

 src/core/above-tab-keycode.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/core/above-tab-keycode.c b/src/core/above-tab-keycode.c
index 56966ab..856afe5 100644
--- a/src/core/above-tab-keycode.c
+++ b/src/core/above-tab-keycode.c
@@ -76,6 +76,8 @@ compute_above_tab_keycode (Display *xdisplay)
   keyboard = XkbGetKeyboard (xdisplay,
                              XkbGBN_ClientSymbolsMask | XkbGBN_KeyNamesMask | XkbGBN_GeometryMask,
                              XkbUseCoreKbd);
+  if (!keyboard)
+    return best_keycode;
 
   geometry = keyboard->geom;