Fix a race condition.

Authentication with valid credentials could be refused in case of concurrent accesses from different users.

PR 63124 [Simon Kappel <simon.kappel axis.com>]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1853190 13f79535-47bb-0310-9956-ffa450edef68