`Perm16::validate` returns false negatives

[james-d-mitchell]

The following happens:

x = Perm16({ 0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 0});
x.validate(8); // returns false

This is counter-intuitive at best.