Prevent changing world players after being set once.

OpenRA.Game/World.cs

@@ -42,6 +42,8 @@ namespace OpenRA
 
 		public void SetPlayers(IEnumerable<Player> players, Player localPlayer)
 		{
+			if (Players.Length > 0)
+				throw new InvalidOperationException("Players are fixed once they have been set.");
 			Players = players.ToArray();
 			SetLocalPlayer(localPlayer);
 		}