Assert that dictionary can own the objects it is given.

core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp

Index: core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp
diff --git a/core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp b/core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp
index cc395a2e089d7634d99331844e59d74a0fac50bf..aeee3382f948326597163f07c7f5e9153e17c436 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_dictionary.cpp
@@ -169,6 +169,7 @@ bool CPDF_Dictionary::IsSignatureDict() const {
 }
 
 void CPDF_Dictionary::SetFor(const CFX_ByteString& key, CPDF_Object* pObj) {
+  ASSERT(!pObj || pObj->GetObjNum() == 0);
   auto it = m_Map.find(key);
   if (it == m_Map.end()) {
     if (pObj)