mod_proxy_ajp: Fix get_content_length().

clength in request_rec is for response sizes, not
request body size.  It is initialized to 0, so the
"if" branch was never taken and thus there's no
functional change (and no CHANGES).

Backport of r1649043 from trunk resp. r1651096 from 2.4.x.

Committed By: rjung
Reviewed By: rjung, ylavic, wrowe
Backported By: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1680815 13f79535-47bb-0310-9956-ffa450edef68