spurious 401s with message "DN has not been defined" when cache expiration...

spurious 401s with message "DN has not been defined" when cache expiration happens in another thread

PR 43786


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@591499 13f79535-47bb-0310-9956-ffa450edef68