Prevent saving after the player has died

This should fix #3056 and #2895. It does not solve the more general problem of crashes after the player has been killed.

It doesn't delete the "_exit" autosave (as suggested in #2895), because I think deleting the user's saves is unfriendly and worse than the possible confusion of returning to an earlier point. It shouldn't even be very confusing -- many games will return you to a previous point if you die.

It would probably be better for the game to continue from the most recent autosave instead of the last '_exit', but that's a larger change than it seems and I don't want to do it for this PR.