fix(multiplayer): validate allocated roster shape

Game/scripts/server_boot.gd

@@ -78,7 +78,7 @@ func _ready() -> void:
 			"server_id": String(config.get_value("server-id")),
 			"protocol": str(NetCodec.PROTOCOL_VERSION),
 			"protocol_version": NetCodec.PROTOCOL_VERSION,
-		}, signing_key):
+		}, signing_key) or MatchNet.assigned_player_slots().size() != roster_tokens.size():
 			printerr("cosmic-clash-server: refusing to start with invalid join-authorisations-file")
 			get_tree().quit(1)
 			return