Investigate key_derive_dh() issue

[Havner]

From yaca-rust tests:

tests/test_key.rs:112

let secret = Key::derive_dh(&key_dh_prv_2, &key_dh_pub).unwrap();
assert_eq!(secret.len(), 256); 

thread 'key_derive' panicked at 'assertion failed: `(left == right)`
  left: `255`,
 right: `256`', tests/test_key.rs:112:5