pkg/tarsum: actually init the TarSum struct

closes #9241

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Add unit test for tarSum.Sum() with no data

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #9248 from vbatts/vbatts-fix_gh9241

pkg/tarsum: actually init the TarSum struct