mpeg_users is always read or written with core->lock held except
in mpeg_release (where it looks like a bug).  A plain int is simpler
and faster.
Tested-by: Andi Huber <hobrom@gmx.at>
Tested-by: Marlon de Boer <marlon@hyves.nl>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>