Guard against double deletion of page views.

fpdfsdk/cpdfsdk_pageview.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 "fpdfsdk/cpdfsdk_pageview.h"
 
 #include <memory>
 #include <vector>
@@ skipping 21 matching lines @@
                                    UnderlyingPageType* page)
     : m_page(page),
       m_pSDKDoc(pSDKDoc),
 #ifndef PDF_ENABLE_XFA
       m_bOwnsPage(false),
 #endif  // PDF_ENABLE_XFA
       m_bEnterWidget(FALSE),
       m_bExitWidget(FALSE),
       m_bOnWidget(FALSE),
       m_bValid(FALSE),
-      m_bLocked(FALSE) {
+      m_bLocked(FALSE),
+      m_bBeingDestroyed(false) {
   CPDFSDK_InterForm* pInterForm = pSDKDoc->GetInterForm();
   if (pInterForm) {
     CPDF_InterForm* pPDFInterForm = pInterForm->GetInterForm();
 #ifdef PDF_ENABLE_XFA
     if (page->GetPDFPage())
       pPDFInterForm->FixPageFields(page->GetPDFPage());
 #else   // PDF_ENABLE_XFA
     pPDFInterForm->FixPageFields(page);
 #endif  // PDF_ENABLE_XFA
   }
@@ skipping 529 matching lines @@
       return pAnnot;
   }
   return nullptr;
 }
 
 int CPDFSDK_PageView::GetPageIndexForStaticPDF() const {
   CPDF_Dictionary* pDict = GetPDFPage()->m_pFormDict;
   CPDF_Document* pDoc = m_pSDKDoc->GetPDFDocument();
   return (pDoc && pDict) ? pDoc->GetPageIndex(pDict->GetObjNum()) : -1;
 }