Avoid a potentially overflowing check

Avoid a potentially overflowing check

This doesn't actually overflow, but still is poor style.

Speaking of which: this is now the second time I get to fix something
reported by Nicky Mouha by way of a blog post. The first time was the
actual SHA-3 buffer overflow in Python where it is not entirely clear
who screwed up and how. Hopefully next time proper communication will
happen and work.

ok jsing