tests: Fix use after free in check_read_indexes
tests: Fix use after free in check_read_indexes

Refs #4376

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <0dc76b2a55bebc49558f30e8d2894973ce817577.1554386770.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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