[EXPLAIN] INSERT .. SELECT creates a select_insert object.
select_insert calls handler->start_bulk_insert() during
initialization.

For MyISAM/Aria this requires that a matching call to
handler->end_bulk_insert() call is made.

Regular INSERT .. SELECT accomplishes this by calling either
select_result->send_eof() or select_result->abort_result_set().

EXPLAIN INSERT ... SELECT didn't call either, which resulted in
improper de-initializaiton of handler object. Make it call
abort_result_set(), which invokes handler->end_bulk_insert().