Use std::map as CPDF_Dictionary's underlying store.

fpdfsdk/src/fpdf_flatten.cpp

Index: fpdfsdk/src/fpdf_flatten.cpp
diff --git a/fpdfsdk/src/fpdf_flatten.cpp b/fpdfsdk/src/fpdf_flatten.cpp
index 76ffec3e26edfbbabb94a8a5c300cad4df18aa35..2a0c2d947c9c087dec0a53a51460512f731e97b1 100644
--- a/fpdfsdk/src/fpdf_flatten.cpp
+++ b/fpdfsdk/src/fpdf_flatten.cpp
@@ -440,10 +440,9 @@ DLLEXPORT int STDCALL FPDFPage_Flatten(FPDF_PAGE page, int nFlag) {
       if (!sAnnotState.IsEmpty()) {
         pAPStream = pAPDic->GetStream(sAnnotState);
       } else {
-        FX_POSITION pos = pAPDic->GetStartPos();
-        if (pos) {
-          CFX_ByteString sKey;
-          CPDF_Object* pFirstObj = pAPDic->GetNextElement(pos, sKey);
+        auto it = pAPDic->begin();
+        if (it != pAPDic->end()) {
+          CPDF_Object* pFirstObj = it->second;
           if (pFirstObj) {
             if (pFirstObj->IsReference())
               pFirstObj = pFirstObj->GetDirect();