compare against undef in a vector crashes the compiler

```
5183	bool const_values_equal(CodeGen *g, ConstExprValue *a, ConstExprValue *b) {
5184	    assert(a->type->id == b->type->id);
5185	    assert(a->special == ConstValSpecialStatic);
5186	    assert(b->special == ConstValSpecialStatic);
```

ignoring `ConstValSpecialUndef`