Merge to XFA: Introduce CPDFDocumentFromFPDFDocument().

fpdfsdk/src/fpdf_dataavail.cpp

Index: fpdfsdk/src/fpdf_dataavail.cpp
diff --git a/fpdfsdk/src/fpdf_dataavail.cpp b/fpdfsdk/src/fpdf_dataavail.cpp
index d3b44160a8104898b88eb1545a0107371ffd9925..f09b173208e7d936c38087e8caedf1d660e17907 100644
--- a/fpdfsdk/src/fpdf_dataavail.cpp
+++ b/fpdfsdk/src/fpdf_dataavail.cpp
@@ -119,10 +119,8 @@ FPDFAvail_GetDocument(FPDF_AVAIL avail, FPDF_BYTESTRING password) {
 }
 
 DLLEXPORT int STDCALL FPDFAvail_GetFirstPageNum(FPDF_DOCUMENT doc) {
-  if (doc == NULL)
-    return 0;
-  CPDF_Document* pDoc = ((CPDFXFA_Document*)doc)->GetPDFDoc();
-  return ((CPDF_Parser*)pDoc->GetParser())->GetFirstPageNo();
+  CPDF_Document* pDoc = CPDFDocumentFromFPDFDocument(doc);
+  return pDoc ? pDoc->GetParser()->GetFirstPageNo() : 0;
 }
 
 DLLEXPORT int STDCALL FPDFAvail_IsPageAvail(FPDF_AVAIL avail,