This affected mainly MyISAM and Aria engines.
Also fixed that end_bulk_insert() detects errors from
internal mi_end_bulk_insert() and ma_end_bulk_insert()

- delete_tree() and delete_tree_element() now has an
  extra argument that marks if future calls to
  tree->free should be ignored.
- tree->free changed to function returning int, to be
  able to signal errors.
- Restored deleting flag in MyISAM that was accidently
  disabled in mi_extra(PREPARE_FOR_DROP)