Fix unnecessary switching of map representation for DiscardUnknownFields

src/google/protobuf/reflection_ops.cc

@@ -327,14 +327,14 @@ void ReflectionOps::DiscardUnknownFields(Message* message) {
       continue;
     }
     // Discard the unknown fields in maps that contain message values.
-    if (field->is_map() && IsMapValueMessageTyped(field)) {
-      const MapFieldBase* map_field =
-          reflection->MutableMapData(message, field);
-      if (map_field->IsMapValid()) {
+    const MapFieldBase* map_field = field->is_map() ?
+        reflection->MutableMapData(message, field) : nullptr;
+    if (map_field != nullptr && map_field->IsMapValid())  {
+      if (IsMapValueMessageTyped(field)) {
         MapIterator iter(message, field);
         MapIterator end(message, field);
         for (map_field->MapBegin(&iter), map_field->MapEnd(&end); iter != end;
-             ++iter) {
+              ++iter) {
           iter.MutableValueRef()->MutableMessageValue()->DiscardUnknownFields();
         }
       }