ap_get_mime_headers_core: alloc space for trailing null when folding

(backport fix for PR18170)

Submitted by: Peter Mayne <PeterMayne@SPAM_SUX.ap.spherion.com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@99445 13f79535-47bb-0310-9956-ffa450edef68