Ticket 458 - Need to properly check if the password admin dn is set
    
    Bug Description:  We were previously checking if password admin sdn was NULL,
                      which it never is.  So it would always search the database
                      looking for the password admins.
    
    Fix Description:  Call slapi_sdn_get_dn() to see if the DN was actually set.
    
    https://fedorahosted.org/389/ticket/458
    
    Reviewed by: richm(Thanks Rich!)