message: go: Fix size comparison for 32-bit systems
  COPYBARA_INTEGRATE_REVIEW=https://github.com/google/tink/pull/579 from rohansingh:master
diff: @@ -73,11 +73,11 @@ func (i *AESGCMInsecureIV) Encrypt(iv, plaintext, associatedData []byte) ([]byte
  return nil, fmt.Errorf("unexpected IV size: got %d, want %d", got, want)
  }
  // Seal() checks plaintext length, but this duplicated check avoids panic.
  - maxPlaintextSize := maxIntPlaintextSize
  + var maxPlaintextSize uint64 = maxIntPlaintextSize
  if maxIntPlaintextSize > aesGCMMaxPlaintextSize {
  maxPlaintextSize = aesGCMMaxPlaintextSize
  }
  - if len(plaintext) > maxPlaintextSize {
  + if uint64(len(plaintext)) > maxPlaintextSize {
  return nil, fmt.Errorf("plaintext too long: got %d", len(plaintext))
  }
  