net: treat ENOTCONN at shutdown as success

While it is not entirely clear why this condition is being
triggered, it does resolve a reported bug.

Fixes: nodejs#26315

PR-URL: nodejs#29912
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/net.js

@@ -36,7 +36,8 @@ const {
 const assert = require('assert');
 const {
   UV_EADDRINUSE,
-  UV_EINVAL
+  UV_EINVAL,
+  UV_ENOTCONN
 } = process.binding('uv');
 
 const { Buffer } = require('buffer');
@@ -357,7 +358,7 @@ Socket.prototype._final = function(cb) {
   req.callback = cb;
   var err = this._handle.shutdown(req);
 
-  if (err === 1)  // synchronous finish
+  if (err === 1 || err === UV_ENOTCONN)  // synchronous finish
     return afterShutdown.call(req, 0);
   else if (err !== 0)
     return this.destroy(errnoException(err, 'shutdown'));