[quanah@freelancer tests]$ *** glibc detected *** /home/quanah/q/openldap-2.4.12/servers/slapd/.libs/lt-slapd: corrupted double-linked list: 0x000000000d28a430 ***

This message comes from glibc malloc. Its data structures are corrupted.

I couldn't figure out much from the the valgrind output. But you'll get more frequent mallocs with CPPFLAGS=-DSLAP_NO_SL_MALLOC, if that code isn't rotted - maybe that'll bring out the problem in another guise. If that code still works, I don't remember.