Fix off-by-one error in msdosfs FAT32 volume label copying

I dropped the + 1 from the other two instances in each file but failed
to do so for this one, resulting in a more egregious buffer overread
than the one I was fixing (since the read character ended up in the
output if there was space).

Reported by: Jenkins
Fixes: 34fb1c133c5b ("Fix intra-object buffer overread for labeled msdosfs volumes")