message: Drop unused variables in utility.cc
  The HMAC() and EVP_DigestFinal() functions accept nullptr
  insted of an int* when the caller doesn't care about the
  length.
diff: @@ -25,8 +25,7 @@ std::vector<uint8_t> Utility::getSha256Digest(const Buffer::Instance& buffer) {
  rc = EVP_DigestUpdate(ctx, slice.mem_, slice.len_);
  RELEASE_ASSERT(rc == 1, "Failed to update digest");
  }
  - unsigned int len;
  - rc = EVP_DigestFinal(ctx, digest.data(), &len);
  + rc = EVP_DigestFinal(ctx, digest.data(), nullptr);
  RELEASE_ASSERT(rc == 1, "Failed to finalize digest");
  EVP_MD_CTX_free(ctx);
  return digest;
  @@ -35,10 +34,9 @@ std::vector<uint8_t> Utility::getSha256Digest(const Buffer::Instance& buffer) {
  std::vector<uint8_t> Utility::getSha256Hmac(const std::vector<uint8_t>& key,
  absl::string_view message) {
  std::vector<uint8_t> hmac(SHA256_DIGEST_LENGTH);
  - unsigned int len;
  const auto ret =
  HMAC(EVP_sha256(), key.data(), key.size(), reinterpret_cast<const uint8_t*>(message.data()),
  - message.size(), hmac.data(), &len);
  + message.size(), hmac.data(), nullptr);
  RELEASE_ASSERT(ret != nullptr, "Failed to create HMAC");
  return hmac;
  }
  