Fix segv in CPDF_DataAvail::CheckRoot() when /Root object is a string.

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 "../../../include/fpdfapi/fpdf_parser.h"
 #include "../../../include/fpdfapi/fpdf_module.h"
 #include "../../../include/fpdfapi/fpdf_page.h"
 #include "../../../src/fxcrt/fx_safe_types.h"
@@ skipping 3372 matching lines @@
         m_docStatus = PDF_DATAAVAIL_LOADALLFILE;
         return TRUE;
     }
     if (!m_pRoot) {
         if (m_docStatus == PDF_DATAAVAIL_ERROR) {
             m_docStatus = PDF_DATAAVAIL_LOADALLFILE;
             return TRUE;
         }
         return FALSE;
     }
-    CPDF_Reference* pRef = (CPDF_Reference*)m_pRoot->GetDict()->GetElement(FX_BSTRC("Pages"));
+    CPDF_Dictionary* pDict = m_pRoot->GetDict();
+    if (!pDict) {
+        m_docStatus = PDF_DATAAVAIL_ERROR;
+        return FALSE;
+    }
+    CPDF_Reference* pRef = (CPDF_Reference*)pDict->GetElement(FX_BSTRC("Pages"));
     if (pRef == NULL || pRef->GetType() != PDFOBJ_REFERENCE) {
         m_docStatus = PDF_DATAAVAIL_ERROR;
         return FALSE;
     }
     m_PagesObjNum = pRef->GetRefObjNum();
     CPDF_Reference* pAcroFormRef = (CPDF_Reference*)m_pRoot->GetDict()->GetElement(FX_BSTRC("AcroForm"));
     if (pAcroFormRef && pAcroFormRef->GetType() == PDFOBJ_REFERENCE) {
         m_bHaveAcroForm = TRUE;
         m_dwAcroFormObjNum = pAcroFormRef->GetRefObjNum();
     }
@@ skipping 1234 matching lines @@
 {
     FX_INT32 iSize = m_childNode.GetSize();
     for (FX_INT32 i = 0; i < iSize; ++i) {
         CPDF_PageNode *pNode = (CPDF_PageNode*)m_childNode[i];
         if (pNode) {
             delete pNode;
         }
     }
     m_childNode.RemoveAll();
 }