mod_proxy_html: Fix variable interpolation and memory allocation failure in ProxyHTMLURLMap.

Proposed by: Ewald Dieterich <ewald mailbox.org>
Reviewed by: ylavic



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1830746 13f79535-47bb-0310-9956-ffa450edef68