Please review 48891 (2nd) : ns-slapd crashes during the shutdown after adding attribute with a matching rule

[thierry bordaz]

The previous fix was commited. It prevented a crash but is incomplete 
because of invalid access to freed buffer 
(https://fedorahosted.org/389/ticket/48891#comment:9).

This is an additional part of the fix
https://fedorahosted.org/389/attachment/ticket/48891/0002-Ticket-48891-ns...