tests/sstable_mutation_test: Don't use moved-from object
tests/sstable_mutation_test: Don't use moved-from object

Fix a bug introduced in dbbb9e93d and exposed by gcc6 by not using a
moved-from object. Twice.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20170802161033.4213-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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