Val::isOneInt failure

## 🐛 Bug

```
    Int* x = new Int(1);
    Int* y = new Int(1);
    Val* z = mul(x, y);
    z->isOneInt(); <-- An exception is thrown here
```
An exception is thrown as:

```
C++ exception with description "bad optional access" thrown in the test body.
```

`Val::isZeroInt` has the same problem.