fix: check if the socket exists in the worker

Reference: https://nodejs.org/api/child_process.html#child_process_example_sending_a_socket_object

> Any 'message' handlers in the subprocess should verify that socket
exists, as the connection may have been closed during the time it takes
to send the connection to the child.

Without this check, the following exception could be thrown:

internal/per_context/primordials.js:23
  return (thisArg, ...args) => ReflectApply(func, thisArg, args);
                               ^

TypeError: Cannot convert undefined or null to object
    at hasOwnProperty (<anonymous>)
    at internal/per_context/primordials.js:23:32
    at getOrSetAsyncId (internal/async_hooks.js:396:7)
    at Server.connectionListener (_http_server.js:414:5)
    at Server.emit (events.js:315:20)

Related: #1

index.js

@@ -99,6 +99,11 @@ const setupWorker = (io) => {
   process.on("message", ({ type, data }, socket) => {
     switch (type) {
       case "sticky:connection":
+        if (!socket) {
+          // might happen if the socket is closed during the transfer to the worker
+          // see https://nodejs.org/api/child_process.html#child_process_example_sending_a_socket_object
+          return;
+        }
         io.httpServer.emit("connection", socket); // inject connection
         // republish first chunk
         if (socket._handle.onread.length === 1) {