storage_service: Add missing return in pieces empty check
storage_service: Add missing return in pieces empty check

If pieces.empty is empty, it is bogus to access pieces[0]:

   sstring move_name = pieces[0];

Fix by adding the missing return.

Spotted by Vlad Zolotarov <vladz@scylladb.com>

Fixes #3258
Message-Id: <9b1d1a3ad4dafaeefd71ce50eea3cc6d23847a1c@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla