Move get form field logic to common method in fpdf_annot.h

Multiple APIs in fpdf_annot.h uses FPDF_FORMHANDLE and FPDF_ANNOTATION
to get the reference of CPDF_FormField. Moving this common logic to a
separate method GetFormField() to avoid duplication.

Change-Id: Ie983b328e7710e7220e429b88cb14e74ee295b19
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/65470
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

fpdfsdk/fpdf_annot.cpp

diff --git a/fpdfsdk/fpdf_annot.cpp b/fpdfsdk/fpdf_annot.cpp
index 2ef525b..923bcff 100644
--- a/fpdfsdk/fpdf_annot.cpp
+++ b/fpdfsdk/fpdf_annot.cpp
@@ -256,6 +256,19 @@
   return pResourceDict;
 }
 
+CPDF_FormField* GetFormField(FPDF_FORMHANDLE hHandle, FPDF_ANNOTATION annot) {
+  CPDF_Dictionary* pAnnotDict = GetAnnotDictFromFPDFAnnotation(annot);
+  if (!pAnnotDict)
+    return nullptr;
+
+  CPDFSDK_InteractiveForm* pForm = FormHandleToInteractiveForm(hHandle);
+  if (!pForm)
+    return nullptr;
+
+  CPDF_InteractiveForm* pPDFForm = pForm->GetInteractiveForm();
+  return pPDFForm->GetFieldByDict(pAnnotDict);
+}
+
 }  // namespace
 
 FPDF_EXPORT FPDF_BOOL FPDF_CALLCONV
@@ -916,16 +929,7 @@
 
 FPDF_EXPORT int FPDF_CALLCONV
 FPDFAnnot_GetFormFieldFlags(FPDF_FORMHANDLE hHandle, FPDF_ANNOTATION annot) {
-  CPDFSDK_InteractiveForm* pForm = FormHandleToInteractiveForm(hHandle);
-  if (!pForm)
-    return FPDF_FORMFLAG_NONE;
-
-  CPDF_Dictionary* pAnnotDict = GetAnnotDictFromFPDFAnnotation(annot);
-  if (!pAnnotDict)
-    return FPDF_FORMFLAG_NONE;
-
-  CPDF_InteractiveForm* pPDFForm = pForm->GetInteractiveForm();
-  CPDF_FormField* pFormField = pPDFForm->GetFieldByDict(pAnnotDict);
+  CPDF_FormField* pFormField = GetFormField(hHandle, annot);
   return pFormField ? pFormField->GetFieldFlags() : FPDF_FORMFLAG_NONE;
 }
 
@@ -955,16 +959,7 @@
 
 FPDF_EXPORT int FPDF_CALLCONV FPDFAnnot_GetOptionCount(FPDF_FORMHANDLE hHandle,
                                                        FPDF_ANNOTATION annot) {
-  CPDFSDK_InteractiveForm* pForm = FormHandleToInteractiveForm(hHandle);
-  if (!pForm)
-    return -1;
-
-  CPDF_Dictionary* pAnnotDict = GetAnnotDictFromFPDFAnnotation(annot);
-  if (!pAnnotDict)
-    return -1;
-
-  CPDF_InteractiveForm* pPDFForm = pForm->GetInteractiveForm();
-  CPDF_FormField* pFormField = pPDFForm->GetFieldByDict(pAnnotDict);
+  CPDF_FormField* pFormField = GetFormField(hHandle, annot);
   return pFormField ? pFormField->CountOptions() : -1;
 }
 
@@ -974,16 +969,10 @@
                          int index,
                          FPDF_WCHAR* buffer,
                          unsigned long buflen) {
-  CPDFSDK_InteractiveForm* pForm = FormHandleToInteractiveForm(hHandle);
-  if (!pForm)
+  if (index < 0)
     return 0;
 
-  CPDF_Dictionary* pAnnotDict = GetAnnotDictFromFPDFAnnotation(annot);
-  if (!pAnnotDict || index < 0)
-    return 0;
-
-  CPDF_InteractiveForm* pPDFForm = pForm->GetInteractiveForm();
-  CPDF_FormField* pFormField = pPDFForm->GetFieldByDict(pAnnotDict);
+  CPDF_FormField* pFormField = GetFormField(hHandle, annot);
   if (!pFormField || index >= pFormField->CountOptions())
     return 0;