media: v4l2-async: Safely unregister an non-registered async subdev

Make the V4L2 async framework a bit more robust by allowing to
unregister a non-registered async subdev. Otherwise the
v4l2_async_cleanup() will attempt to delete the async subdev from the
subdev_list with the corresponding list_head not initialized.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/v4l2-core/v4l2-async.c

@@ -813,6 +813,9 @@ EXPORT_SYMBOL(v4l2_async_register_subdev);
 
 void v4l2_async_unregister_subdev(struct v4l2_subdev *sd)
 {
+	if (!sd->async_list.next)
+		return;
+
 	mutex_lock(&list_lock);
 
 	__v4l2_async_notifier_unregister(sd->subdev_notifier);