Fix setting mission stats

[MjnMixael]

I think when I originally tested this I used the return value to verify that it was working. Turns out that wasn't a good enough test because the scoring_struct_h explicitly makes a copy of the player stats, probably to prevent manipulation. However, in the case of storing and reloading mission stats (for checkpoints), we need to actually apply the stats to the player's stats and not the copied stats.

So I've added a way to get the player from the scoring stats and in the case of setting the mission stats, apply them directly so that it actually.. ya know.. works.