Fix a segfault when requests for shared memory fails and returns

NULL. Fix a segfault caused by a lack of bounds checking on the
cache.
PR:	24801
Obtained from:
Submitted by:
Reviewed by:	minfrin, bnicholes, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@103760 13f79535-47bb-0310-9956-ffa450edef68