ICE with slice in a generic struct

The following code causes an internal compiler error:

```
fn foo<T>() {
    let _x: Bar<T> = Bar{ arr: &[] };
}

struct Bar<T> {
    arr: &[T],
}

fn test_foo() {
    foo::<()>();
}
```

Switching the `&[T]` to a `Option<T>` makes it go away.

Backtrace here: https://gist.github.com/3924593
