Game does not start if a client leaves while the game is synchronizing

[elexis]

If the host detects the disconnect of a client while the game is in the "Waiting for other players to connect" state (synchronizing), then the game will never start and has to be abolished.

Reproduce:

1. Start a host with one instance
2. Join with a second instance via the actual internet IP
3. Click the start game button with the host
4. Immediately thereafter simulate infinite lag (ticket:3264#comment:5) on the internet connection (but not the localhost one)

After the loading screen vanished, a chat message appears that the client disconnected and the game will never start.

[elexis]

In 18858:

Fix a bug where the networked game would never start if a client would disconnect while synchronizing, fixes #4289.