mod_mem_cache: Fix concurrent removal of stale entries which could lead

to a crash. 

PR: 43724
Submitted by: ylavic
Reviewed by: covener, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1750846 13f79535-47bb-0310-9956-ffa450edef68