Some times we may close an empty active segment, if all data in it was
evicted. Normally segments are removed as soon as the last object in
it is freed, but if the segment is already empty when closed, noone is
supposed to call free on it. Such segments would be quickly reclaimed
during compaction, but it's possible that we will destroy the region
before they're reclaimed by compaction. Currently we would fail on an
assertion which checks that there are no segments. This change fixes
the problem by handling empty closed segments when region is
destroyed.
3caad2294b