In commit 8ea923f5 (MDEV-24818)
when we optimized multi-statement INSERT transactions into empty tables,
we would roll back the entire transaction on any error. But, we would
fail to invalidate any SAVEPOINT that had been requested in the past.

trx_t::savepoints_discard(): Renamed from trx_roll_savepoints_free().

row_mysql_handle_errors(): If we were in bulk insert, invoke
trx_t::savepoints_discard(). In this way, a future attempt of
ROLLBACK TO SAVEPOINT will return an error.