Guard against double deletion of page views.

fpdfsdk/fpdfview.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 "public/fpdfview.h"
 
 #include <memory>
 #include <utility>
@@ skipping 652 matching lines @@
 DLLEXPORT void STDCALL FPDF_ClosePage(FPDF_PAGE page) {
   UnderlyingPageType* pPage = UnderlyingFromFPDFPage(page);
   if (!page)
     return;
 #ifdef PDF_ENABLE_XFA
   pPage->Release();
 #else   // PDF_ENABLE_XFA
   CPDFSDK_PageView* pPageView =
       static_cast<CPDFSDK_PageView*>(pPage->GetView());
   if (pPageView) {
+    // We're already destroying the pageview, so bail early.
+    if (pPageView->IsBeingDestroyed())
+      return;
+
     if (pPageView->IsLocked()) {
       pPageView->TakePageOwnership();
       return;
     }
 
     bool owned = pPageView->OwnsPage();
     // This will delete the |pPageView| object. We must cleanup the PageView
     // first because it will attempt to reset the View on the |pPage| during
     // destruction.
     pPageView->GetSDKDocument()->RemovePageView(pPage);
@@ skipping 441 matching lines @@
   if (!buffer) {
     *buflen = len;
   } else if (*buflen >= len) {
     memcpy(buffer, utf16Name.c_str(), len);
     *buflen = len;
   } else {
     *buflen = -1;
   }
   return (FPDF_DEST)pDestObj;
 }