Ticket 628 - crash in aci evaluation
    
    Bug Description:  When trying to get effective rights on a entry that does not exist,
                      can lead to a crash from dereferencing a NULL pointer.
    
    Fix Description:  Check for NULL entry pointers and return the appropriate error.
    
    https://fedorahosted.org/389/ticket/628
    
    Reviewed by: nhosoi(Thanks Noriko!)