Fix port forwarder resource leak (https://github.com/rootless-containers/rootlesskit/issues/153).
Changes: https://github.com/rootless-containers/rootlesskit/compare/v0.9.5...v0.10.0
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 5bc41368d9dd8e020732e4cd9fcbc8231a9b75b2)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>