Update util.py

The string will exceed in length if we add 4 zero to the string.
Thus, we could only add extra zero according to the length.

I have check some relevant information.If adding four zeroes would make the string longer than it should be, though, then we would only add the number of zeroes necessary to make the string that it is the correct length. So In this case, we should only add the number of zeroes necessary to make sure that it is the correct length.


beakey zhjie