Fix off-by-one error in fstyp(8) and geom_label(4) that made them use
a single space (" ") as a CD9660 label name when no label was present.
Similar problem was also present in msdosfs label recognition.

PR: 200828
Differential Revision: https://reviews.freebsd.org/D2830
Reviewed by: asomers@, emaste@
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation