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 by: ylavic, rpluem, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1576714 13f79535-47bb-0310-9956-ffa450edef68