async-hooks,net: ensure asyncId=null if no handle

If the .listen() hasn't been called on the server, there is no handle
object. In this case use null as the triggerAsyncId.

Fixes: #13548
PR-URL: #13938
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>

lib/net.js

@@ -1558,7 +1558,8 @@ Server.prototype.getConnections = function(cb) {
   const self = this;
 
   function end(err, connections) {
-    nextTick(self[async_id_symbol], cb, err, connections);
+    const asyncId = self._handle ? self[async_id_symbol] : null;
+    nextTick(asyncId, cb, err, connections);
   }
 
   if (!this._usingSlaves) {

test/async-hooks/test-net-get-connections.js

@@ -0,0 +1,11 @@
+'use strict';
+
+const common = require('../common');
+const net = require('net');
+const server = net.createServer();
+
+// This test was based on an error raised by Haraka.
+// It caused server.getConnections to raise an exception.
+// Ref: https://github.com/haraka/Haraka/pull/1951
+
+server.getConnections(common.mustCall());