TopicPartitionList::elements violates unsafe precondition in slice::from_raw_parts_mut

[hadronzoo]

The following panics on rebalance when compiled with Rust 1.78.0:

impl ConsumerContext for Context {
    fn pre_rebalance(&self, rebalance: &Rebalance) {
        if let Rebalance::Assign(assigned) = rebalance {
            for element in assigned.elements() {
                // never reached
            }
        }
    }
}

Panic message:

unsafe precondition(s) violated: slice::from_raw_parts_mut requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`

It appears to be caused by the slice::from_raw_parts_mut call in TopicPartitionList::elements:

    pub fn elements(&self) -> Vec<TopicPartitionListElem<'_>> {
        let slice = unsafe { slice::from_raw_parts_mut((*self.ptr).elems, self.count()) };
        let mut vec = Vec::with_capacity(slice.len());
        for elem_ptr in slice {
            vec.push(TopicPartitionListElem::from_ptr(self, &mut *elem_ptr));
        }
        vec
    }

It looks like elems can be null:

If I check elements capacity and return if it's 0, this panic does not occur:

            if assigned.capacity() == 0 {
                return;
            }

            for element in assigned.elements() {
                // does not panic

It appears that TopicPartitionList::elements should check that cnt is greater than zero before calling slice::from_raw_parts_mut.