mod_{proxy,authnz}_fcgi: follow up to r1750392.

Don't try to read past AP_FCGI_END_REQUEST on the backend side, it would be
a response for the next request.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1755930 13f79535-47bb-0310-9956-ffa450edef68