Incorrect unreachable pattern when matching slice with ..

Code:

```
fn foo(v: &[int]) {
    match v {
        [_] => {},
        [_, ..] => {},
        _ => {},
    }
}
```

Wrong error is:

```
/home/vagrant/tmp2.rs:4:9: 4:16 error: unreachable pattern
/home/vagrant/tmp2.rs:4         [_, ..] => {},
                                ^~~~~~~
error: aborting due to previous error
```
