BitSet.bitand operation returns non-empty BitSetAnd even if no ids match.

Code:

```
use hibitset::*;

fn main() {
let mut a = BitSet::new();
let mut b = BitSet::new();
assert!( a.is_empty() && b.is_empty(), "This shouldn't happen!");
a.add(0);
b.add(10);
assert!( !a.is_empty() && !b.is_empty(), "This also shouldn't happen!");
assert!( (a&b).is_empty(), "This definetely shouldn't happen!");
}
```

Output:
```
thread 'main' panicked at 'This definetely shouldn't happen!', src\main.rs:10:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: process didn't exit successfully: `target\debug\rust-test.exe` (exit code: 101)
```