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)