Ensure that length argument of memcmp() isn't seen as negative.

Ensure that length argument of memcmp() isn't seen as negative.

I think this will shut up a weird warning from buildfarm member
serinus.  Perhaps it'd be better to change tsCompareString's
length arguments to unsigned, but that seems more invasive
than is justified.

Part of a general push to remove off-the-beaten-track warnings
where we can easily do so.