progress bar: don't strings.Repeat a negative number

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
jsonmessage: added test and cleaned up others

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #7274 from vbatts/vbatts-gh7136

progress bar: don't strings.Repeat a negative number