Cleanup the client to backend brigade before returning an error (if any) to...

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.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1572543 13f79535-47bb-0310-9956-ffa450edef68