Prevent the OpenSSL id_callback from pointing at a mod_ssl

function after mod_ssl is unloaded.

* ssl_util.c (ssl_util_thread_cleanup): Clear the id_callback.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100767 13f79535-47bb-0310-9956-ffa450edef68