subtract with overflow in new line iterator

It appears there is an issue that was introduced by #70.

The issue is an subtract with overflow in https://github.com/cessen/ropey/blob/1029afac9ff0878cb5a68152745fa855dd996d62/src/iter.rs#L1240

The following code reproduces the issue.

```rust
use ropey::Rope;

fn main() {
    dbg!(Rope::from_str("").lines_at(0).collect::<Vec<_>>());
}
```
