fix potential overflow

[mirceanis]

see dchest/tweetnacl-js#187

[Artoria2e5]

This change should not be needed. It was needed for JS because every JS bitwise operation is preceded by a 32-bit signed integer conversion, but by all means (x[j] + 128) should be a long, representing a 64-bit signed integer in Kotlin. The original analysis in the cited issue suggests that the worse case is requiring 33 bits to store the number, so 64 should be more than needed.

(Take this comment with every grain of salt you have though. Between phlegm and caffeine, I'm not the person with the clearest head right now...)

[mirceanis]

Interesting.

Also, thanks for reminding me about this lib. I had forgotten about it :)

It's unmaintained