Bring in upstream commit ec165c392ca54317dbe3064a8c200de6531e89ad:
Unregister the KEXINIT handler after message has been
received. Otherwise an unauthenticated peer can repeat the KEXINIT and cause
allocation of up to 128MB -- until the connection is closed. Reported by
shilei-c at 360.cn
Security: CVE-2016-8858