Incorrect check for memory safety in presence of closures

With rust 0.6, the following code produces a segfault. Indeed, at the time of println, *y is no longer a managed pointer to a string, but an arbitrary value crafted by the closure.

```
enum A { B(@str), C(int) }
fn main() -> () {
  let mut x = B(@"");
  let f = || { x = C(0); };
  match x {
    B(ref y) => { f(); println(*y) },
    C(*) => ()
  }
}
```
