Merge r431339 from trunk:

PR#37770: Don't try to use dead backend connection in proxy
(Patch from Olivier BOEL)

Submitted by: niq
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@472104 13f79535-47bb-0310-9956-ffa450edef68