Fix code style and errors

src/playlist/playlistbackend.cpp

@@ -291,7 +291,7 @@ void PlaylistBackend::SavePlaylist(int playlist, const PlaylistItemList &items,
   if (db_->CheckErrors(clear)) return;
 
   // Save the new ones
-  for (PlaylistItemPtr item : items) {
+  for (const PlaylistItemPtr &item : items) {
     insert.bindValue(":playlist", playlist);
     item->BindToQuery(&insert);