Message formatting: handle unexpected case of failing property lookup.

src/messages.cc

Index: src/messages.cc
diff --git a/src/messages.cc b/src/messages.cc
index fd10d45d4a02c59f9c43dc5b1a9c7f4c446bf289..a8afa3f5a75067c76c173d301c3d41b0a789a4bc 100644
--- a/src/messages.cc
+++ b/src/messages.cc
@@ -449,6 +449,7 @@ MaybeHandle<String> ErrorToStringHelper::Stringify(Isolate* isolate,
 bool ErrorToStringHelper::ShadowsInternalError(
     Isolate* isolate, LookupIterator* property_lookup,
     LookupIterator* internal_error_lookup) {
+  if (!property_lookup->IsFound()) return false;
   Handle<JSObject> holder = property_lookup->GetHolder<JSObject>();
   // It's fine if the property is defined on the error itself.
   if (holder.is_identical_to(property_lookup->GetReceiver())) return true;