Re: [PATCH 1/3] pack-objects: fix tree_depth and layer invariants

> But in (b), we use the number of stored objects, _not_ the allocated
> size of the objects array. So we can run into a situation like this:
>
>   1. packlist_alloc() needs to store the Nth object, so it grows the
>      objects array to M, where M > N.
>
>   2. oe_set_tree_depth() wants to store a depth, so it allocates an
>      array of length N. Now we've violated our invariant.
>
>   3. packlist_alloc() needs to store the N+1th object. But it _doesn't_
>      grow the objects array, since N <= M still holds. We try to assign
>      to tree_depth[N+1], which is out of bounds.