dbcheck: Allow a dangling forward link outside our known NCs

dbcheck: Allow a dangling forward link outside our known NCs

If we do not have the NC of the target object we can not be really sure
that the object is redundent and so we want to keep it for now
and not (as happened until now) break the dbcheck run made during the
replication stage of a "samba-tool domain backup rename".

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14450

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>