radar.lua: fix error when playing multiple games in the same session

- Because the player variable is cached at file scope, it becomes orphaned on end game.
- Reset it during cleanup phase so it can be populated with the correct Game.player object during the next game.

data/pigui/modules/radar.lua

@@ -506,6 +506,7 @@ end)
 
 -- reset radar to default at game end
 Event.Register("onGameEnd", function()
+	player = nil
 	shouldDisplay2DRadar = false
 	radar2d:resetZoom()
 	radar3d:resetZoom()