Return 0 on a non-matching kdf_type

If we have a non-matching kdf_type then pkey_dh_derive silently succeeds.
It should fail. This is a "should not happen" condition anyway so the
impact is negligible.

Fixes #2440

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6000)