sporadically

Under some circumstances, the mysql_insert_id() value after SELECT ...
INSERT could return a wrong value. This could happen when the last
SELECT ... INSERT did not involve an AUTO_INCREMENT column, but the
value of mysql_insert_id() was changed by some previous statements.

Fixed by checking the value of thd->insert_id_used in
select_insert::send_eof() and returning 0 for mysql_insert_id() if it
is not set.