fix: fix gcm tag length truncation

[DasDingGehtNicht]

the truncate method takes the count of the number of bytes to trim off as an argument.
the calculation to calculate this number should therefore be oldLengthInBytes - (wantedLengthInBits / 8)

e.g. the original tag is 16 bytes long, i specify a tagLength of 32 bits, so the tag should be 4 bytes.
old calculation: 16 % (32/8) = 0 => no truncation => 16 bytes tag length
new calculation: 16 - (32/8) = 12 => truncate 12 bytes from the 16 bytes => 4 bytes tag length