Ticket 47620 - Fix dereferenced NULL pointer in agmtlist_modify_callback()
    
    The server would dereference a NULL point if an attribute was deleted from a replication
    agreement.
    
    https://fedorahosted.org/389/ticket/47620
    
    Reviewed by: rmeggins(Thanks!)
    (cherry picked from commit 8baed897f504e75478b5dbbe736c1eaf6d2d7fa9)
    (cherry picked from commit 60d263f7bc52e4b5186a01c38868763a275abadc)