ipa-range-check: do not treat missing objects as error
    
    Currently the range check plugin will return a 'Range Check error'
    message if a ldapmodify operation tries to change a non-existing object.
    Since the range check plugin does not need to care about non-existing
    objects we can just return 0 indicating that the range check plugin has
    done its work.
    
    Resolves https://fedorahosted.org/freeipa/ticket/4924
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>