message: Fix check for allocator_stats
diff: @@ -207,7 +207,7 @@ void UpdateStats(OpKernelContext* context,
  absl::optional<AllocatorStats> allocator_stats =
  allocator_pair.first->GetStats();
  - if (stats) {
  + if (allocator_stats) {
  memory->set_allocator_bytes_in_use(allocator_stats->bytes_in_use);
  }
  allocator_pair.second->GetRecordsAndUnRef();
  