[media] videodev: fix a NULL pointer dereference in v4l2_device_release()

The change in 8280b662 does not cover the case when v4l2_dev is already
NULL, fix that.

With a Kinect sensor, seen as an USB camera using GSPCA in this context,
a NULL pointer dereference BUG can be triggered by just unplugging the
device after the camera driver has been loaded.
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-dev.c

@@ -181,7 +181,7 @@ static void v4l2_device_release(struct device *cd)
 	 * TODO: In the long run all drivers that use v4l2_device should use the
 	 * v4l2_device release callback. This check will then be unnecessary.
 	 */
-	if (v4l2_dev->release == NULL)
+	if (v4l2_dev && v4l2_dev->release == NULL)
 		v4l2_dev = NULL;
 
 	/* Release video_device and perform other