Before the fix: 
  ha_partition objects had ha_partition::m_part_info==NULL and that caused
  crash
After: 
- The new ha_partition::clone() function makes the clones use parent's
  m_part_info value.
- The parent ha_partition object remains responsible for deallocation of
  m_part_info.


mysql-test/r/partition_innodb.result:
  BUG#26117 "index_merge sort-union over partitioned table crashes"
   - Testcase
mysql-test/t/partition_innodb.test:
  BUG#26117 "index_merge sort-union over partitioned table crashes"
   - Testcase