checker: fix missing check for interface cast of option type

[felipensp]

Fix missing interface cast checking

interface IFoo {
	a int
}

struct Foo {
	a int
}

fn main() {
	a := ?Foo{}
	dump(a)
	b := IFoo(a?)
	c := IFoo(a) // must be an error
}

[spytheman]

Excellent work.