bug 44136: Don't rollback after successful commit

For PostgreSQL only:

SavepointPostgres tries to rollback already committed/rolled
back transactions, so after it got committed, don't try
to rollback it again in SavepointPostgres::__destroy.

This happens only when trying to INSERT IGNORE something
without having a transaction open first (e.g. during
unit testing).

Thanks to: OverlordQ <overlordq@gmail.com>

Change-Id: I40ff757a7e2c6ff21f73e1ecd35754e1981f1941
Merge "bug 44136: Don't rollback after successful commit"