- the problem was caused by EXPLAIN INSERT SELECT. For that statement, 
  the code would call select_insert::prepare2(), which would call 
  handler->ha_start_bulk_insert().  The corresponding handler->end_bulk_insert() 
  call is made from select_insert::send_eof or select_insert::abort_result_set 
  which are never called for EXPLAIN INSERT SELECT.
- Fixed by re-using approach of mysql-5.6: don't call ha_start_bulk_insert() if 
  we are in EXPLAIN.