I have not reproduced your exact problem. However I did find [1] which
seems possibly relevant, and I can reproduce the original bug with ldclt
as in [2] in Debian (with slapd as server). I also experimented with a
slapd config containing 5 syncrepl clients, all using GSSAPI, and
encountered a couple of occurrences of memory corruption.
Besides the RH bug in cyrus-sasl, the lack of concurrency protection
around libldap's sasl_client_init still concerns me, too; so there are a
couple of avenues to look into.
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1147659
[2] https://pagure.io/389-ds-base/issue/47800
