media: v4l: Safely to call v4l2_subdev_cleanup on an uninitialised subdev

Graciously handle an uninitialised (but still zeroed) sub-device in
v4l2_subdev_cleanup(). The list_empty() check there is unnecessary, too,
so replace that by cheking whether the lists's next field is NULL.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>