dounmount: Avoid &((struct vnode_impl *)NULL)->vi_vnode.

Member access of a null pointer is undefined, even if the result
should also be null because vi_vnode is at the start of vnode_impl.

Reported-by: syzbot+a4b2d13c0d6d4dac2d07@syzkaller.appspotmail.com
https://syzkaller.appspot.com/bug?extid=a4b2d13c0d6d4dac2d07

(riastradh)