vme4l: fix null pointer dereference when exitin module

vme4l-core module is trying to access a null pointer on exit with vme4l_cleanup_module(). This G_bDrv pointer is assigned to NULL previously with vme4l_unregister_bridge_driver().

Fix this by protecting the callback function G_bDrv->releaseAddrWindow as this may get called via an ioctl and not just when doing the module cleanup.

fixes: 22d760a6 ("vme4l-core.c: applied patch from M.Suminski to release address windows")