s3/locking: Fix assertion check on lock reference count

s3/locking: Fix assertion check on lock reference count

lock_ref_count will always hold the old value prior to change. Thus it
would mean that if lock_ref_count is 0 the new value is already -1 which
is not expected here. Therefore it is better to make sure that it is
always greater than 0 rather than >= 0.

Signed-off-by: Anoop C S <anoopcs@redhat.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Jul 31 04:23:47 CEST 2018 on sn-devel-144