row_cache: Fix handling of concurrent partition population
row_cache: Fix handling of concurrent partition population

This fixes a regression introduced in 27a3b4bca9643 (master only).

partition_range_cursor assumes that as long as references are valid,
_end is valid as well. But if new entries were inserted before _end,
it may not, if the new entries fall after the query range. This may
result in reads returning partitions from outside the query range.
Message-Id: <4d6b9f4802993ae5cd8ecf4c09e01b9204e7d1a4@scylladb.com>

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