Handle ap_discard_request_body() being called more than once.

Add a debug assertion to verify that c->remain is zero when a new
request starts.  ap_http_filter() does the wrong thing otherwise.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88540 13f79535-47bb-0310-9956-ffa450edef68