Val::isOneInt failure

[naoyam]

🐛 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.