Deal with broken _NET_WORKAREA more gracefully

fvwm seems to have problems keeping _NET_WORKAREA in sync with
the number of desktops. Instead of reading garbage, silently use
the full screen as workarea for desktops that are not covered
by the _NET_WORKAREA property.

https://bugzilla.gnome.org/show_bug.cgi?id=698248

gdk/x11/gdkscreen-x11.c

@@ -359,6 +359,8 @@ get_work_area (GdkScreen    *screen,
     goto out;
 
   desktop = get_current_desktop (screen);
+  if (desktop + 1 > num / 4) /* fvwm gets this wrong */
+    goto out;
 
   workareas = (long *) ret_workarea;
   area->x = workareas[desktop * 4];