(ITS#7798) back-mdb can crash if slapadd runs with slapd

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.4.39
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (178.197.231.190)
Submitted by: hyc


back-mdb is supposed to work for slapd/slapadd etc all running concurrently.
slapd can crash if slapadd runs and the newly added entries use attributes that
weren't in the database when slapd was started, because mdb_entry_decode()
doesn't check for attribute validity.

A fix is coming shortly.