Backport: Fix the global mutex crash when the global mutex is never allocated

due to disabled/empty caches.
PR:
Obtained from:
Submitted by:	Jess Holle
Reviewed by:	minfrin, bnicholes, trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105270 13f79535-47bb-0310-9956-ffa450edef68