clang::Comment::num_children should bounds check its argument

[fitzgen]

The Clang docs don't specify what happens if we request a child comment that is out of bounds. We should make sure we don't do it at all. We can make the function return an Option<Comment> instead of plain Comment and return None if i >= self.num_children(). Otherwise we can proceed normally and return Some.

I can mentor whoever would like to work on this.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[catdesk]

I'd like to work on this, but is it actually Comment::get_child that needs to be modified?