obtain partition number, call partition_info->get_part_partition_id() when
the table has subpartitions, and get_partition_id() otherwise. (The bug
was that we were always doing the latter)


mysql-test/r/partition_pruning.result:
  Testcase for BUG#18659
mysql-test/t/partition_pruning.test:
  Testcase for BUG#18659