Crashes in one-player mode when the next played map in the queue is not installed

[kimden]

Description

See above.

While the server crash is related to absent check for track existence, client crash is probably related to some logical error in the code that allows to start selection even if the player doesn't have the track, just because of specific mode.

Steps to reproduce

1. Use the server in one-player mode
2. Push a map not installed on your client and/or server to the queue
3. Start selection. If the server doesn't have the map installed, server timeouts.
4. Otherwise, select a kart. If the client doesn't have the map installed, client crashes.

[kimden]

Thanks to FabianF for noticing: https://youtu.be/SY2oZXtD4aI