private int readBlock() throws IOException {
    int blockSize = readAndWriteNextByte();
    int n = 0;
    if (blockSize > 0) {
      while (n < blockSize) {
        n += readIntoBlock(n, blockSize - n);
      }
    }
    return n;
  }
@param is InputStream to decode
@param os OutputStream to write fixed version of gif, if needed. (optional)
animated-gif-lite/src/main/java/com/facebook/animated/giflite/decoder/GifMetadataDecoder.java