Fix autocubemap crashing when not in a level

[Blixibon]

This fixes a bug in which running autocubemap_start would crash the game if there is no map or map list loaded.

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Does this PR close any issues?

• [MISC] Autocubemap crashes when not in a level #296

PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
• My PR targets a develop branch OR targets another branch with a specific goal in mind

[1upD]

One question about this - launching the game with -dev doesn't load the main menu background map, right? Will that case reach this code, or is this only after the first level in the map list is loaded?

[1upD]

I left a comment with a question but aside from my question this looks good.