A proc can be called twice when captured in a proc

``` rust
fn main() {
    let a = ~"foo";
    let f = proc() { println!("{}", a); drop(a) };
    (proc() {
        f();
        f();
    })()
}
```

understandably crashes on the second `f` call.

Seems similar to #12041 (although no loop required), and #10398.
