fix stack overflow when sending huge request body to upstream (CVE-2017-10869)

[kazuho]

The worker process of H2O may crash (and automatically respawned depending on the configuration) while the reverse proxy module tries to forward a huge HTTP request body to the upstream server using HTTPS.

The crash disrupts other requests in-flight, and therefore is being classified as a DoS vulnerability.

Details TBD.

Affected systems: H2O up to version 2.2.2, used as a reverse proxy that connects to the origin server using HTTPS.
Resolution: upgrade to 2.2.3.

[NicoleG25]

@kazuho Was this vulnerability ever addressed ? and if so in what commit was this fixed ?