Aphlict - fix getActiveListenerCount return value

Summary: Ref T7126. Dictionaries do not have a `length` property unlike arrays resulting in the `getActiveListenerCount()` function returning undefined results. Using the `length` property on the array of keys will work.

Test Plan: Using `wscat` to generate multiple connections to the server, establish new ones and close others while keeping an eye on the displayed `clients.active` value.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7126

Differential Revision: https://secure.phabricator.com/D11647

Author: pierremoreau

support/aphlict/server/lib/AphlictListenerList.js

@@ -42,7 +42,7 @@ JX.install('AphlictListenerList', {
     },
 
     getActiveListenerCount: function() {
-      return this._listeners.length;
+      return Object.keys(this._listeners).length;
     },
 
     getTotalListenerCount: function() {