Clean up signed vs. unsigned warnings.

Clean up signed vs. unsigned warnings.

If length is a uint8_t, subtracting 2 (a value of type int) from it
yields a value of type int, so compariing it against a u_int index
causes a signed vs. unsigned warning.

Make it a u_int instead (before we subtract 2 from it, we've already
checked to make sure it's >= 2, so we know the result will never be
negative).