Doesn't forbid moving into owned closure multiple times

``` rust
fn main() {
    let h = ~[1,2,3];
    do std::task::try {
        println(fmt!("%?", h));
    };
    do std::task::try {
        println(fmt!("%?", h));
    };
}
```

This causes a segfault. Also happens if you put the `do` in a loop.
