partition_snapshot_row_cursor: Clear current row when recreating
partition_snapshot_row_cursor: Clear current row when recreating

The cursor keeps current row in a separate vector of iterators
and reconstructs it in a dedicated method, which _expects_ that
the vector is empty on entry.

It's better to keep the logic of current row construction in one
place.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla