Test case for already-fixed NPE with value classes

This progressed along with the fix for #66.

`TreeGen.mkZero` is a bit of a minefield: first with `Nothing` and
now with Value Classes. I wonder if we can provoke the same sort of
bug in the compiler in places where this is used.

Closes #83
Merge pull request #85 from retronym/ticket/83

Test case for already-fixed NPE with value classes