code: public static byte[] toUnsignedByteArray(int length, BigInteger value) {
  		byte[] bytes = value.toByteArray();
  		if (bytes.length == length) {
  			return bytes;
  		}
  
  		int start = bytes[0] == 0 ? 1 : 0;
  		int count = bytes.length - start;
  
  		if (count > length) {
  			throw new IllegalArgumentException("standard length exceeded for value");
  		}
  
  		byte[] tmp = new byte[length];
  		System.arraycopy(bytes, start, tmp, tmp.length - count, count);
  		return tmp;
  	}
docs: 以无符号字节数组的形式返回传入值。
  
  @param length bytes长度
  @param value 需要转换的值
  @return 无符号bytes
  @since 4.5.0