BUG/MAJOR: http: fix the 'next' pointer when performing a redirect

Commit bed410e ("MAJOR: http: centralize data forwarding in the request path")
has woken up an issue in redirects, where msg->next is not reset when flushing
the input buffer. The result is an attempt to forward a negative amount of
data, making haproxy crash.

This bug does not seem to affect versions prior to dev23, so no backport is
needed.
1 file changed