Incorrect "non-exhaustive patterns" message with zero-variant enum

```
enum what { }

fn what_to_str(x: what) -> ~str
{
    alt x {
    }
}

fn main()
{
}
```

gives me

```
/Users/jruderman/Desktop/unlikely_non-exhaustive.rs:5:4: 6:5 error: non-exhaustive patterns
/Users/jruderman/Desktop/unlikely_non-exhaustive.rs:5     alt x {
/Users/jruderman/Desktop/unlikely_non-exhaustive.rs:6     }
```

I did exhaust all the cases of the `what` enum, however vacuously :)
