Fix segv above CPDFSDK_Document::GetPageView()

fpdfsdk/src/fpdfformfill.cpp

Index: fpdfsdk/src/fpdfformfill.cpp
diff --git a/fpdfsdk/src/fpdfformfill.cpp b/fpdfsdk/src/fpdfformfill.cpp
index d21d5d209dd37be5bfb7863edc5fcf5b1e1e314d..42e94c789db6cb61e0fe17b9bd3e2a902b185837 100644
--- a/fpdfsdk/src/fpdfformfill.cpp
+++ b/fpdfsdk/src/fpdfformfill.cpp
@@ -27,6 +27,9 @@ CPDFSDK_InterForm* FormHandleToInterForm(FPDF_FORMHANDLE hHandle)
 
 CPDFSDK_PageView* FormHandleToPageView(FPDF_FORMHANDLE hHandle, FPDF_PAGE page)
 {
+    if (!page)
+        return nullptr;
+
     CPDFSDK_Document* pSDKDoc = FormHandleToSDKDoc(hHandle);
     return pSDKDoc ? pSDKDoc->GetPageView((CPDF_Page*)page, TRUE) : nullptr;
 }