util/base64: fix padding bytes for trailing data

util/base64: fix padding bytes for trailing data

Padding bytes for the last remainder data should be as follows:

Case   |    Remainder bytes     |    Padding
----------------------------------------------
  I    |              1         |      3
  II   |              2         |      2
  III  |              3         |      1

However, we calculate the decoded_bytes with the formula:
decoded_bytes = ASCII_BLOCK - padding

this means for Case I when padding is 3 bytes, the decoded_bytes would
be 0. This is incorrect for any trailing data. In any of the above
cases, if the parsing was successful, there should at least be 1 decoded
byte.