cryptnet: Sign-compare warning fix (try 2)

[Juan Lang]

> That's the trouble with some warnings: some instances you want to fix and
> some you know you can safely ignore. I presumed that we were aiming to turn
> this warning on permanently, but that may not be the intention, and maybe
> the downside you highlight makes doing so not worthwhile.

Turning on -Wsign-compare may be useful, but I don't think combining
it with -Werror is.  This is the trouble with warnings in general:
they may or may not point out a real bug.  Fixing the bugs is useful,
but fixing non-bugs really isn't, unless the code is somehow improved
as a result.  Making code look worse to remove a warning is clearly
not useful.
--Juan