Use the LookupIterator for SetElement and friends

src/api.h

Index: src/api.h
diff --git a/src/api.h b/src/api.h
index 6d7aaeab4adec09d6404310a530c842be86c82fe..b20ef5cf66f3cb401875ea40c1a3fca8b3e8045e 100644
--- a/src/api.h
+++ b/src/api.h
@@ -95,6 +95,7 @@ void NeanderObject::set(int offset, v8::internal::Object* value) {
 
 template <typename T> inline T ToCData(v8::internal::Object* obj) {
   STATIC_ASSERT(sizeof(T) == sizeof(v8::internal::Address));
+  if (obj == v8::internal::Smi::FromInt(0)) return nullptr;
   return reinterpret_cast<T>(
       reinterpret_cast<intptr_t>(
           v8::internal::Foreign::cast(obj)->foreign_address()));
@@ -105,6 +106,7 @@ template <typename T>
 inline v8::internal::Handle<v8::internal::Object> FromCData(
     v8::internal::Isolate* isolate, T obj) {
   STATIC_ASSERT(sizeof(T) == sizeof(v8::internal::Address));
+  if (obj == nullptr) return handle(v8::internal::Smi::FromInt(0), isolate);
   return isolate->factory()->NewForeign(
       reinterpret_cast<v8::internal::Address>(reinterpret_cast<intptr_t>(obj)));
 }