gdk: Don't crash when resetting cursor

When the toplevel is a GdkOffscreenWindow which doesn't
implement the set_device_cursor() vfunc.

https://bugzilla.gnome.org/show_bug.cgi?id=675809

gdk/gdkwindow.c

@@ -7805,6 +7805,7 @@ update_cursor (GdkDisplay *display,
    * which native window has what cursor set. */
   toplevel = get_event_toplevel (pointer_window);
   impl_class = GDK_WINDOW_IMPL_GET_CLASS (toplevel->impl);
+  if (impl_class->set_device_cursor)
     impl_class->set_device_cursor (toplevel, device, cursor);
 }