dbcheck: don't remove dangling one-way links on already deleted objects

dbcheck: don't remove dangling one-way links on already deleted objects

This would typically happen when the garbage collection
removed a parent object before a child object (both with
the DISALLOW_MOVE_ON_DELETE bit set in systemFlags),
while dbcheck is running at the same time as the garbage collection.
In this case the lastKnownParent attributes points a non existing
object.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13816

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit e388e599495b6d7c38b8b6966332e27f8b958783)