Test allocation of memory beforehand if we are trying to return a > 2 MB BLOB; normally InnoDB asserts if memory allocation fails
ha_innodb.cc:
  Do not fetch all columns if change_active_index() is called during a query; a sum(a), max(a) query seemed to do that, doing unnecessary copying (the change actually made in the previous bk ci)
  Free BLOB heap of handle when MySQL calls some ::extra()'s