mod_proxy_http: follow up to r1656259.
The proxy connection may be NULL during prefetch, don't try to dereference it!
Still origin->keepalive will be set according to p_conn->close by the caller
(proxy_http_handler).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1678771 13f79535-47bb-0310-9956-ffa450edef68
