packet lib: don't crash with truncated dhcp packet

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
rest_router: fix security problem

Signed-off-by: Takeshi <a86487817@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>