haproxy: denial of service

A problem has been discovered with the new field "rule_deny_status"
into struct http_txn, which is filled only by actions "http-request
deny" and "http-request tarpit". It’s then used in the deny code path
to emit the proper error message, but is used uninitialized when the
deny comes from a "reqdeny" rule, causing random behaviours ranging
from returning a 200, an empty response, or crashing the process.