message: Clear a copied secret key after negation
  Summary: This is a backport of secp256k1 PR619
  Test Plan:
  make check
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -458,6 +458,7 @@ int secp256k1_ec_privkey_negate(const secp256k1_context* ctx, unsigned char *sec
  secp256k1_scalar_negate(&sec, &sec);
  secp256k1_scalar_get_b32(seckey, &sec);
  + secp256k1_scalar_clear(&sec);
  return 1;
  }
  