Merge r1853190 from trunk:

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

PR: 63124
Submitted by: Simon Kappel <simon.kappel axis.com>
Reviewed by: jailletc36, icing, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1855298 13f79535-47bb-0310-9956-ffa450edef68