hfs: add sanity check for file name length

On a corrupted file system the ->len field could be wrong leading to
a buffer overflow.

Reported-and-acked-by: Clement LECIGNE <clement.lecigne@netasq.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>