mod_disk_cache: Do not store aborted content.

PR: 21492
Submitted by:	R�diger Pl�m <r.pluem t-online.de>
Reviewed by:	Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@105317 13f79535-47bb-0310-9956-ffa450edef68