message: SerializeIterator: do not unref the resource until we're finished using it.
  This change avoids a potential use-after-free error if the resource is concurrently serialized and destroyed (e.g. by a DestroyResourceOp or Session::Reset()).
diff: @@ -787,7 +787,7 @@ class SerializeIteratorOp : public OpKernel {
  IteratorResource* iterator_resource;
  OP_REQUIRES_OK(
  ctx, LookupResource(ctx, HandleFromInput(ctx, 0), &iterator_resource));
  - iterator_resource->Unref();
  + core::ScopedUnref unref_iterator(iterator_resource);
  Tensor* variant_t;
  OP_REQUIRES_OK(ctx, ctx->allocate_output(0, TensorShape({}), &variant_t));
  IteratorStateVariant v;
  