I can still reproduce this pretty soon when running this test with several iterations.
testrun/slapd.3.log ends with:
connection_close: conn=1014 sd=9 =>ldap_back_conn_destroy: fetching conn 1014 =>ldap_back_conn_destroy: fetching conn 1014 =>ldap_back_conn_destroy: fetching conn 1014 conn=1014 fd=9 closed (connection lost) daemon: shutdown requested and initiated. slapd shutdown: waiting for 0 operations/tasks to finish lt-slapd shutdown: initiated lt-slapd destroy: freeing system resources. ldap_free_connection 1 1 ldap_send_unbind ber_flush2: 7 bytes to sd 13 ldap_free_connection: actually freed ldap_free_connection 1 1 ldap_send_unbind ber_flush2: 7 bytes to sd 11 ldap_free_connection: actually freed ldap_free_connection 1 1 ldap_send_unbind ber_flush2: 7 bytes to sd 12 ldap_free_connection: actually freed ldap_free_connection 1 1 ldap_send_unbind ber_flush2: 7 bytes to sd 10 ldap_free_connection: actually freed slapd stopped. lt-slapd: at.c:242: at_delete_names: Assertion `air != ((void *)0)' failed.