mod_mem_cache: Don't cache incomplete responses when the client

connection is aborted before the body is fully read.  PR 45049.

Backports: n/a (2.2.x only)
Submitted by: Nick Pace <nick simplylogic.net>, Edward Lu, Yann Ylavic
Reviewed by: ylavic, wrowe, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1756565 13f79535-47bb-0310-9956-ffa450edef68