A proc can be called twice when captured in a proc

[huonw]

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.

[alexcrichton]

Nominating.

[flaper87]

cc @flaper87

[pnkfelix]

Assigning 1.0, P-backcompat-lang.