The bug was that for ordered index scans, ha_partition::index_init() did
not put index columns into table->read_set if the underlying storage 
engine did not have HA_PARTIAL_COLUMN_READ flag. 
This was causing assertion failure when handle_ordered_index_scan() tried
to sort the records according to index order.

Fixed by making ha_partition::index_init() put index columns into table->read_set
for all ordered scans.