async_wrap: add a missing case to test-async-wrap-throw-no-init

PR-URL: https://github.com/nodejs/node/pull/8198
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

test/parallel/test-async-wrap-throw-no-init.js

@@ -4,11 +4,14 @@ require('../common');
 const assert = require('assert');
 const async_wrap = process.binding('async_wrap');
 
-
 assert.throws(function() {
   async_wrap.setupHooks(null);
 }, /first argument must be an object/);
 
+assert.throws(function() {
+  async_wrap.setupHooks({});
+}, /init callback must be a function/);
+
 assert.throws(function() {
   async_wrap.enable();
 }, /init callback is not assigned to a function/);