cluster: fix closing dgram sockets in cluster workers throws errors

This fixes closing dgram sockets right after binding in cluster
workers will throws `ERR_SOCKET_DGRAM_NOT_RUNNING` errors.

lib/internal/cluster/child.js

@@ -111,6 +111,10 @@ cluster._getServer = function(obj, options, cb) {
   });
 
   obj.once('listening', () => {
+    // short-lived sockets might have been closed
+    if (!indexes.has(indexesKey)) {
+      return;
+    }
     cluster.worker.state = 'listening';
     const address = obj.address();
     message.act = 'listening';

test/parallel/test-dgram-cluster-close-in-listening.js

@@ -0,0 +1,29 @@
+'use strict';
+// Ensure that closing dgram sockets in 'listening' callbacks of cluster workers
+// won't throw errors.
+
+const common = require('../common');
+const dgram = require('dgram');
+const cluster = require('cluster');
+if (common.isWindows)
+  common.skip('dgram clustering is currently not supported on windows.');
+
+if (cluster.isPrimary) {
+  for (let i = 0; i < 3; i += 1) {
+    cluster.fork();
+  }
+} else {
+  const socket = dgram.createSocket('udp4');
+
+  socket.on('error', common.mustNotCall());
+
+  socket.on('listening', common.mustCall(() => {
+    socket.close();
+  }));
+
+  socket.on('close', common.mustCall(() => {
+    cluster.worker.disconnect();
+  }));
+
+  socket.bind(0);
+}