mod_proxy_http: Ensure that when the backend is closed, the final call to

ap_proxy_http_cleanup() is suppressed, so that the backend is not cleaned
twice. Fixes a further thread safety issue.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1055246 13f79535-47bb-0310-9956-ffa450edef68