Fixes #2173
Per-shard min positions can be unset if we never collected any
sstable/truncation info for it, yet replay segments of that id.
Wrap the lookups to handle "missing data -> default", which should have been
there in the first place.
Message-Id: <1490185101-12482-1-git-send-email-calle@scylladb.com>
(cherry picked from commit c3a510a08d)