Vec::swap_remove fails if the vector is empty.

Sample code:

``` rust
fn main() {
    let mut x: Vec<int> = vec!();
    let y = x.swap_remove(0);
    println!("{}", y);
}
```

Expected to print `None`, but instead it fails with:

```
task '<main>' failed at 'index out of bounds: the len is 0 but the index is 0', src/test.rs:1
```

rustc version: 0.12.0-pre.
