zero pad DHE public key in ServerKeyExchange message for interop

Some versions of the Microsoft TLS stack have problems when the DHE public key
is encoded with fewer bytes than the DHE prime. (Backported from master)

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1350)