Previously, sml_iterator() could not free the last segment while iterating,
because it could have turned out to be over-allocated at any time and be
removed.

With the change from the previous commit, we can now lift this limitation and
take a reference to also the last segment.

This is relevant because, due to of the race between the fetch and iterator site
of a transient operation, the "is last segment" condition can happen at any
point while an object is busy.