[media] media-entity: clear media_gobj.mdev in _destroy()

[media] media-entity: clear media_gobj.mdev in _destroy()

media_gobj_destroy() may be called twice on one instance - once by
media_device_unregister() and again by dvb_media_device_free().  The
function media_remove_intf_links() establishes and documents the
convention that mdev==NULL means that the object is not registered,
but nobody ever NULLs this variable.  So this patch really implements
this behavior, and adds another mdev==NULL check to
media_gobj_destroy() to protect against double removal.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>