Fix for Bug 21287.

Patch extracted and modified from attachment in Bug 21285.
The submitted patch was combining fix for 21285 and 21287.
It is extending the mutex protection in decrement_refcount() and remove_url().


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102215 13f79535-47bb-0310-9956-ffa450edef68