sheep: check unknown join result in sd_join_handler()

sd_join_handler() does nothing even if a join result is unknown
value. The unknown value means sd_check_join_cb() is buggy, so
after receiving such a value sheep should print error message and die.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>