Division by zero when running solver

The following set of constraints crashes:

```haskell
crash :: Maybe [Defined Integer]
crash = Control.Monad.Watson.satisfying (from 2 [0, 1]) $ \[x, y] ->
  x .== lift 1
  .&&
    (x .*. y) .== y

```

with the error

```
*** Exception: divide by zero
```