mod_auth_ldap: Fix some segfaults in the cache logic.

PR:                18756
Submitted by:      Matthieu Estrade <apache@moresecurity.org>
Reviewed by:       Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101706 13f79535-47bb-0310-9956-ffa450edef68