sheep: make invalid_vnode() type safe

Comparing sd_node and sd_vnode with node_id_cmp should be avoided.  It
is because node_id must be the first field of both structures but we
don't have any checks for it.

This also fixes a compile error in debug build.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>