gdkdisplay-x11: Do not pass a NULL source device to _gdk_display_device_grab_update

Pass the master device instead if the last slave is NULL. This is
unlikely to happen in most of the cases, but can happen when running
unit tests where there's no pointer interaction to update the last
slave.

https://bugzilla.gnome.org/show_bug.cgi?id=696756

gdk/x11/gdkdisplay-x11.c

@@ -1737,7 +1737,8 @@ device_grab_update_callback (GdkDisplay *display,
 
   pointer_info = _gdk_display_get_pointer_info (display, device);
   _gdk_display_device_grab_update (display, device,
-                                   pointer_info->last_slave, serial);
+                                   pointer_info->last_slave ? pointer_info->last_slave : device,
+                                   serial);
 }
 
 #define XSERVER_TIME_IS_LATER(time1, time2)                        \