Fix segfault when attempting to generate or enter in an invalid game variant.

Summary:
This change resolves T5981.
The segfault is fixed by checking whether or not a Game object is valid, before letting the user play with it.

Test Plan:
Manually added a borked game variant in KSudoku::updateShapesList() using a CustomGameVariant with null QUrl.
Compiled and verified the segfault no longer occurred.

```
/*
 * For the purpose of testing the change, a deliberately borked game variant.
 * TODO: remove in final version of diff.
 */
variant = new CustomGame(i18n("Invalid TODO Remove me"), QUrl(), m_gameVariants);
variant->setDescription(i18n("Invalid TODO Remove me"));
variant->setIcon("ksudoku-ksudoku_9x9");
```

Reviewers: #kde_games, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, #kde_games

Maniphest Tasks: T5981

Differential Revision: https://phabricator.kde.org/D5671