g_label: Handle small sector sizes when tasting

Make sure that the provider sector size is large enough to contain a
valid label before trying to read it. We performed this check already
for most label types, but not for several filesystem labels.

Reported by: syzbot+f52918174cdf193ae29c@syzkaller.appspotmail.com
MFC after: 1 week
Sponsored by: The FreeBSD Foundation