format_commonmark panics on a single list item

[silverpill]

Panic occurs when format_commonmark input contains a single NodeValue::Item:

https://github.com/kivikakk/comrak/blob/v0.29.0/src/cm.rs#L468

It didn't panic before version 0.25.0 (unwrap was introduced in #429).

[kivikakk]

That's an interesting/unexpected use case! I suppose we can revert to the previous behaviour if the stack is indeed unexpectedly empty:

let list_number = if let Some(last_stack) = self.ol_stack.last_mut() {
    let ln = *last_stack;
    if entering {
        *last_stack += 1;
    }
    ln
} else {
    match node.data.borrow().value {
        NodeValue::Item(ref ni) => ni.start,
        NodeValue::TaskItem(_) => parent.start,
        _ => unreachable!(),
    }
};

What do you think? Please feel free to open a PR; it'd also be great to have a test with your use case in mind so it doesn't regress!

[silverpill]

That solution works. I'll submit a PR shortly