Use apr_pcalloc for rewritemap_entry struct, to avoid uninitialized entries.

PR: 53663
Submitted by: Mikhail T. <mi apache aldan algebra com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1369995 13f79535-47bb-0310-9956-ffa450edef68