Add some nullptr checks that were missing from commit f7992b39.

core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp

 // Copyright 2014 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 <set>
 #include <utility>
 #include <vector>
 
@@ skipping 834 matching lines @@
                       if (!pRoot ||
                           (pRef &&
                            (FX_DWORD)m_CrossRef.GetSize() >
                                pRef->GetRefObjNum() &&
                            m_CrossRef.GetAt(pRef->GetRefObjNum()) != 0)) {
                         FX_POSITION pos = pTrailer->GetStartPos();
                         while (pos) {
                           CFX_ByteString key;
                           CPDF_Object* pElement =
                               pTrailer->GetNextElement(pos, key);
-                          FX_DWORD dwObjNum = pElement->GetObjNum();
+                          FX_DWORD dwObjNum =
+                              pElement ? pElement->GetObjNum() : 0;
                           if (dwObjNum) {
                             m_pTrailer->SetAtReference(key, m_pDocument,
                                                        dwObjNum);
                           } else {
                             m_pTrailer->SetAt(key, pElement->Clone());
                           }
                         }
                         pObj->Release();
                       } else {
                         pObj->Release();
@@ skipping 4080 matching lines @@
   if (!m_pLinearizedDict)
     return -1;
   CPDF_Array* pRange = m_pLinearizedDict->GetArray(FX_BSTRC("H"));
   if (!pRange)
     return -1;
   CPDF_Object* pStreamLen = pRange->GetElementValue(1);
   if (!pStreamLen)
     return -1;
   return pStreamLen->GetInteger();
 }