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

fixes #7136

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
@vbatts @vieux Couldn't we just do `if percentage < 0 { percentage = 0 }` and `if percentage > 50 { percentage = 50 }` ?