net.websocket: Fix traceback in case of ondisconnect being called twice

We want to figure out what situations the double ondisconnect happens in, and
aim to fix the root cause in the future.

net/websocket.lua

@@ -23,6 +23,7 @@ local websockets = {};
 local websocket_listeners = {};
 function websocket_listeners.ondisconnect(conn, err)
 	local s = websockets[conn];
+	if not s then return; end
 	websockets[conn] = nil;
 	if s.close_timer then
 		timer.stop(s.close_timer);