Merge r1572543 from trunk:

Cleanup the client to backend brigade before returning an error (if any) to avoid buckets lifetime issues (backend connection's pool destroyed before request's one). PR 50335.
Suggested by rpluem.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1573359 13f79535-47bb-0310-9956-ffa450edef68