Fix for Clusterfuzz issue 343928.

test/mjsunit/regress/regress-343928.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony --expose-debug-as=debug
+
+(function () {  // Scope for utility functions.
+  escaping_function = function(object) {
+    // Argument must not be null or undefined.
+    var string = Object.prototype.toString.call(object);
+    // String has format [object <ClassName>].
+    return string.substring(8, string.length - 1);
+  }
+})();
+
+module B {
+  var stuff = 3
+}
+
+var __v_0 = {};
+var __v_4 = debug.MakeMirror(__v_0);
+print(__v_4.referencedBy().length);  // core dump here if not fixed.