Check for nullptrs in CPDF_Dictionary dtor.

core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "core/fpdfapi/fpdf_parser/include/cpdf_dictionary.h"
 
+#include <set>
+#include <utility>
+
 #include "core/fpdfapi/fpdf_parser/cpdf_boolean.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_array.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_name.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_number.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_reference.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_stream.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_string.h"
 #include "third_party/base/stl_util.h"
 
 CPDF_Dictionary::CPDF_Dictionary() {}
 
 CPDF_Dictionary::~CPDF_Dictionary() {
+  // Mark the object as deleted so that it will not be deleted again
+  // in case of cyclic references.
   m_ObjNum = kInvalidObjNum;
-  for (const auto& it : m_Map)
-    it.second->Release();
+  for (const auto& it : m_Map) {
+    if (it.second)
+      it.second->Release();
+  }
 }
 
 CPDF_Object::Type CPDF_Dictionary::GetType() const {
   return DICTIONARY;
 }
 
 CPDF_Dictionary* CPDF_Dictionary::GetDict() const {
   // The method should be made non-const if we want to not be const.
   // See bug #234.
   return const_cast<CPDF_Dictionary*>(this);
@@ skipping 215 matching lines @@
                                   const CFX_Matrix& matrix) {
   CPDF_Array* pArray = new CPDF_Array;
   pArray->AddNumber(matrix.a);
   pArray->AddNumber(matrix.b);
   pArray->AddNumber(matrix.c);
   pArray->AddNumber(matrix.d);
   pArray->AddNumber(matrix.e);
   pArray->AddNumber(matrix.f);
   SetAt(key, pArray);
 }