db: properly handle a reduction in arch filters on transaction commits

Fix an off-by-one error that was causing us to leak a db_filter struct
at transaction commit time when we removed an arch/ABI in a transaction.

Reported-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>