SECURITY (CVE-2014-3581): Fix a mod_cache NULL pointer deference

in Content-Type handling.

mod_cache: Avoid a crash when Content-Type has an empty value. PR56924.

Submitted By: Mark Montague <mark catseye.org>
Reviewed By: Jan Kaluza


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1624234 13f79535-47bb-0310-9956-ffa450edef68