storage_proxy: fix _is_short_read computation
storage_proxy: fix _is_short_read computation

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