Buffer::nth() resolved the requested position by adding it to the first row key
and indexing the rows array by that computed key. That assumes contiguous keys,
but Buffer::delete() removes rows with unset() without reindexing, leaving gaps.
After a deletion nth() therefore returned the wrong record (or an empty array for
a valid position, or a record for an out-of-range position). Resolve the position
against the record order with array_values() so nth() matches getRecords() and
ResultSet::nth().
