Introduce CPDF_Document::FromFPDFDocument().

fpdfsdk/src/fpdf_dataavail.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/fpdf_dataavail.h"
 #include "../../public/fpdf_formfill.h"
 #include "../include/fsdk_define.h"
 
@@ skipping 97 matching lines @@
     delete pParser;
     ProcessParseError(err_code);
     return NULL;
   }
   ((CFPDF_DataAvail*)avail)->m_pDataAvail->SetDocument(pParser->GetDocument());
   CheckUnSupportError(pParser->GetDocument(), FPDF_ERR_SUCCESS);
   return pParser->GetDocument();
 }
 
 DLLEXPORT int STDCALL FPDFAvail_GetFirstPageNum(FPDF_DOCUMENT doc) {
-  if (doc == NULL)
+  CPDF_Document* pDoc = CPDF_Document::FromFPDFDocument(doc);
+  if (!doc)
     return 0;
-  CPDF_Document* pDoc = (CPDF_Document*)doc;
   return ((CPDF_Parser*)pDoc->GetParser())->GetFirstPageNo();
 }
 
 DLLEXPORT int STDCALL FPDFAvail_IsPageAvail(FPDF_AVAIL avail,
                                             int page_index,
                                             FX_DOWNLOADHINTS* hints) {
   if (avail == NULL || hints == NULL)
     return 0;
   CFPDF_DownloadHintsWrap hints_wrap(hints);
   return ((CFPDF_DataAvail*)avail)
       ->m_pDataAvail->IsPageAvail(page_index, &hints_wrap);
 }
 
 DLLEXPORT int STDCALL FPDFAvail_IsFormAvail(FPDF_AVAIL avail,
                                             FX_DOWNLOADHINTS* hints) {
   if (avail == NULL || hints == NULL)
     return -1;
   CFPDF_DownloadHintsWrap hints_wrap(hints);
   return ((CFPDF_DataAvail*)avail)->m_pDataAvail->IsFormAvail(&hints_wrap);
 }
 
 DLLEXPORT FPDF_BOOL STDCALL FPDFAvail_IsLinearized(FPDF_AVAIL avail) {
   if (avail == NULL)
     return -1;
   return ((CFPDF_DataAvail*)avail)->m_pDataAvail->IsLinearizedPDF();
 }