Problem: when alter to partitioned table,
it does not see it as change of engine.
Solution: If alter includes partitioning, check if it is possible
to change engines (eg. is the table referenced by a FK)
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.
