(ITS#8843) null modlist with MMR > 2 can cause segv

Full_Name: Quanah Gibson-Mount Version: 2.4.46 OS: Linux URL: ftp://ftp.openldap.org/incoming/ Submission from: (NULL) (47.208.148.239)

There is a race condition with MMR >2 that can cause slapd to segv, due to the op->modlist being set to NULL for a change that's already been processed.

If the target entry is already newer than the mod, and we then look in the logDB to see what was changed, and the newer mod is a delete(attr) then any other changes to that attr are dropped from the modlist, which can result in the modlist being NULL