query_pagers: Fix dereference of potentially disengaged _last_ckey optional
query_pagers: Fix dereference of potentially disengaged _last_ckey optional

Message-Id: <5c88f3ff1e9c0302b61d685d2b7b932caf011e60@scylladb.com>

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