Can you confirmed you've actually tried this?

Because i found 2 places where if you would have tried it, it would have not worked, so i'm a bit puzzled as to how much testing this patch got at all

Yes, I have tried it. At first I just launched the game and checked if the functionality worked. After your last comment I deleted all files and checked all scenarios I could think of from the start. I'm sorry that I didn't do it before I submitted this diff. I didn't think about the initial state of the game.

Also, I have replied to both of your comments inline. I can't see my comments, though, when I'm not logged in. Don't know if you can see them when you're logged in (unintuitive, if they somehow are not visible for logged in users). Here they are:

You're correct. When the game is first launched after the update the starting value would be 0 instead of -1 and would never be updated. Thank you.
You're right again. Sorry that I didn't reread the entire code after your first comment. Reread it now and changed 1 more thing. I couldn't find a scenario, though, in which this 0 would be a problem (but of course better safe than sorry with -1).