Fix the case where if there is no ErrorDocument specified for an error,

the error would be sent to the client *twice* because both the filter
and the core would trigger error responses.

The problem is that the filters have already handled some errors (say 413)
and due to the ap_get_client_block API, the error was morphed into 400.
Therefore, ap_discard_request_body must use brigades directly rather than
the ap_get_client_block API so that any potential errors are not dropped.

The special value AP_FILTER_ERROR indicates that the lower level has
already dealt with this problem (ap_die() will realize this).  Otherwise,
we'll error with HTTP_BAD_REQUEST and ap_die() will take it from there.

This also prevents needless memory copies when we are just going to
discard it anyway.

Thanks to Cliff Woolley who found this wacky problem.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95424 13f79535-47bb-0310-9956-ffa450edef68