MDEV-26582 SIGSEGV in spider_db_bulk_insert and spider_db_connect and spider_db_before_query, and hang in "End of update loop" / "Reset for next command" query states

Spider accesses a freed connection in ha_spider::end_bulk_insert()
and results in SIGSEGV.

The cause of the bug is that ha_spider::is_bulk_insert_exec_period()
wrongly returns TRUE when the bulk insertion has not yet started.

Spider decides whether it is during the bulk insertion or not by
the value of insert_pos, but the variable is not reset in a case,
and this result in the bug.