[HttpFoundation] smaller fixes for PdoSessionHandler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10652
| License       | MIT

- https://github.com/symfony/symfony/pull/10652#issuecomment-42370425
- https://github.com/symfony/symfony/pull/10652#issuecomment-44359784