sstables: mutation_reader: Avoid index lookups when out of range
sstables: mutation_reader: Avoid index lookups when out of range

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