Introduce CPDF_Document::FromFPDFDocument().

fpdfsdk/src/fpdfformfill.cpp

Index: fpdfsdk/src/fpdfformfill.cpp
diff --git a/fpdfsdk/src/fpdfformfill.cpp b/fpdfsdk/src/fpdfformfill.cpp
index 37af1fbf958876b04f06e4053a9c9942a8f691fd..fed9a7e97e5429fd8c8c67ff9be800e850cfb155 100644
--- a/fpdfsdk/src/fpdfformfill.cpp
+++ b/fpdfsdk/src/fpdfformfill.cpp
@@ -79,10 +79,12 @@ DLLEXPORT int STDCALL FPDFPage_FormFieldZOrderAtPoint(FPDF_FORMHANDLE hHandle,
 DLLEXPORT FPDF_FORMHANDLE STDCALL
 FPDFDOC_InitFormFillEnvironment(FPDF_DOCUMENT document,
                                 FPDF_FORMFILLINFO* formInfo) {
-  if (!document || !formInfo || formInfo->version != 1)
+  if (!formInfo || formInfo->version != 1)
     return nullptr;
 
-  CPDF_Document* pDocument = (CPDF_Document*)document;
+  CPDF_Document* pDocument = CPDF_Document::FromFPDFDocument(document);
+  if (!pDocument)
+    return nullptr;
   CPDFDoc_Environment* pEnv = new CPDFDoc_Environment(pDocument, formInfo);
   pEnv->SetSDKDocument(new CPDFSDK_Document(pDocument, pEnv));
   return pEnv;