In the ha_partition::position() we don't calculate the number
of the partition of the record, but use m_last_part value instead,
relying on that it's previously set by some other call like ::write_row().
Delete_rows_log_event::do_exec_row() calls find_and_fetch_row(),
where we used position() + rnd_pos() call for the InnoDB-based PARTITION-ed
table as there HA_PRIMARY_KEY_REQUIRED_FOR_POSITION enabled.

fixed by introducing new handler::rnd_pos_by_record() method to be
used for random record-based positioning