Apply upstream fix:

Skip passwords longer than 1k in length so clients can't
easily DoS sshd by sending very long passwords, causing it to spend CPU
hashing them. feedback djm@, ok markus@.

Brought to our attention by tomas.kuthan at oracle.com, shilei-c at
360.cn and coredump at autistici.org

Security: CVE-2016-6515
Security: FreeBSD-SA-17:06.openssh