rgw: RGWRados::get_obj() returns wrong len if len == 0

Fixes: #9877
We only updated if len was > 0, should update it if r >= 0. This was the
culprit for issue #9877.
Backport: giant, firefly

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2859 from ceph/wip-9877

rgw: RGWRados::get_obj() returns wrong len if len == 0

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>