dlist.split_off(dlist.len()) should not panic

[mbrubeck]

Currently, Dlist::split_off requires its argument to be strictly less than the length of the list, which means the following function panics when passed a one-element list. Instead, it should leave the original list unmodified and return an empty list:

fn split_head<T>(list: &mut DList<T>) -> DList<T> {
    list.split_off(1)
}

In contrast, Vec::split_off does allow self.len() as a valid argument, so the above code works as desired if you replace DList with Vec.

[Gankra]

Vec was actually like this until a few days ago when I happened to notice it and change the check. I must've mis-specified it in the RFC, or this went viral from the first mis-write.