accesslog moving from 2.4. to 2.5 SIGSEGV on startup

When slapcat-ing a 2.4 accesslog database and slapadd-ing it to a 2.5 instance, slapd will segfault on startup (specifically logpurge) due to a missing minCSN, which was added in 2.5 [1].

Though I haven't done too much analysis on the correct fix, 2 options are:

1. Add the minCSN to your accesslog LDIF. I simply used the entryCSN of the accesslog suffix. 2. Check that li->li_mincsn is not null in log_old_lookup(). Simple patch attached.

I attached a minimal slapd conf and accesslog.ldif to duplicate the issue.

[1] https://git.openldap.org/openldap/openldap/-/commit/66a743f119c85ee83f2efcdd...