Change move_is_ok() and square_is_ok() in something useful

Change move_is_ok() and square_is_ok() in something useful

As is defined now is always true, tested with:

  for (long i=-1000000; i < 1000000; i++)
      if (!move_is_ok(Move(i)))
          exit(0);

Reason is that move_from() and move_to() already truncate the
input value to something in the range [0, 63] that is always
a possible square.

So change definition to something useful.

The same applies also to square_is_ok()

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>