Merge r1618401 from trunk:

mod_proxy_fcgi: Fix faulty logging of large amounts of stderr from the
application.

PR: 56858
Submitted by: Manuel Mausz <manuel-asf mausz.at>
Reviewed by: trawick

Submitted by: trawick
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1621602 13f79535-47bb-0310-9956-ffa450edef68