test: increase coverage for lib/events.js

Adds tests for the case in which listeners() is invoked on a
EventEmitter with no events.

Adds a new test case for the situation in which a class
inherits from the EventEmitter but overrides the constructor
in the EventEmitter so that the _events is never set.

PR-URL: nodejs#9865
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

test/parallel/test-event-emitter-listeners.js

@@ -3,9 +3,12 @@
 require('../common');
 const assert = require('assert');
 const events = require('events');
+const util = require('util');
 
 function listener() {}
 function listener2() {}
+class TestStream { constructor() { } }
+util.inherits(TestStream, events.EventEmitter);
 
 {
   const ee = new events.EventEmitter();
@@ -49,3 +52,14 @@ function listener2() {}
   ee.once('foo', listener2);
   assert.deepStrictEqual(ee.listeners('foo'), [listener, listener2]);
 }
+
+{
+  const ee = new events.EventEmitter();
+  ee._events = undefined;
+  assert.deepStrictEqual(ee.listeners('foo'), []);
+}
+
+{
+  const s = new TestStream();
+  assert.deepStrictEqual(s.listeners('foo'), []);
+}