main/openssh: curve25519pad patch added

https://lists.mindrot.org/pipermail/openssh-unix-dev/2014-April/032494.html:
> bad bignum encoding for curve25519-sha256@libssh.org
>[...]
> So I screwed up when writing the support for the curve25519 KEX method
> that doesn't depend on OpenSSL's BIGNUM type - a bug in my code left
> leading zero bytes where they should have been skipped. The impact of
> this is that OpenSSH 6.5 and 6.6 will fail during key exchange with a
> peer that implements curve25519-sha256@libssh.org properly about 0.2%
> of the time (one in every 512ish connections).