Fix crash in meta_display_get_above_tab_keycode

https://bugzilla.gnome.org/show_bug.cgi?id=647777

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;