[wmii] Bug(?) with selecting pages out of range

If you select a page out of range (e.g. alt+shift+9 when there isn't
a tenth page) you end up on page 0. I kind of like this as it's often
quicker to get to page 0 but none the less it's unexpected.

A patch that fixes this is enclosed. I don't really like how I've done
it, but I can't think of a better way.

--
Adam Gleave