hmm, just realised that this only solves half of the problem. In fact, it’s a bad solution, so don’t do that 
Better to prevent it going out of range in the first place by adding conditionals in the button bar actions. So you’d need to make each of them custom actions and put the logic in there.
On the left (decrement) side for example, maybe something like:
- If val > 0 then increment by -1
- Else Open Link “#”
(The “None” action isn’t available inside custom actions, so the “Open Link #” is a trick you can use to simulate one.)