isofs: Fix out of bound access for corrupted isofs image

When isofs image is suitably corrupted isofs_read_inode() can read data
beyond the end of buffer. Sanity-check the directory entry length before
using it.

Reported-and-tested-by:  <syzbot+6fc7fb214625d82af7d1@syzkaller.appspotmail.com>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>