row_cache: Fix single_partition_populating_reader not waiting on create_underlying() to resolve
row_cache: Fix single_partition_populating_reader not waiting on create_underlying() to resolve

Results in undefined behavior.
Message-Id: <a224db62e882656a9870b66dfd8474ca335f83a6@scylladb.com>

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