debugger: assert test before accessing this.binding

PR-URL: https://github.com/nodejs/node/pull/5145
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_debug_agent.js

@@ -48,6 +48,7 @@ function Agent() {
 
   this.first = true;
   this.binding = process._debugAPI;
+  assert(this.binding, 'Debugger agent running without bindings!');
 
   var self = this;
   this.binding.onmessage = function(msg) {
@@ -57,7 +58,6 @@ function Agent() {
   };
 
   this.clients = [];
-  assert(this.binding, 'Debugger agent running without bindings!');
 }
 util.inherits(Agent, net.Server);
 

test/parallel/test-debug-agent.js

@@ -0,0 +1,6 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+assert.throws(() => { require('_debug_agent').start(); },
+  assert.AssertionError);