code: private void checkReadableBytesUnsafe(int minimumReadableBytes) {
  		if (readerIndex > writerIndex - minimumReadableBytes) {
  			throw new IndexOutOfBoundsException(
  					String.format("readerIndex(%d) + length(%d) exceeds writerIndex(%d): %s", readerIndex,
  							minimumReadableBytes, writerIndex, this));
  		}
  	}
docs: 检查是否可读字节数
  @param minimumReadableBytes