Throw an exception at load when a map has more than 64 players

OpenRA.Game/Map/Map.cs

@@ -331,6 +331,9 @@ namespace OpenRA
 				throw new InvalidDataException("Map format {0} is not supported.\n File: {1}".F(MapFormat, package.Name));
 
 			PlayerDefinitions = MiniYaml.NodesOrEmpty(yaml, "Players");
+			if (PlayerDefinitions.Count > 64)
+				throw new InvalidDataException("Maps must not define more than 64 players.\n File: {0}".F(package.Name));
+
 			ActorDefinitions = MiniYaml.NodesOrEmpty(yaml, "Actors");
 
 			Grid = modData.Manifest.Get<MapGrid>();