ryu/app/rest_firewall.py: handling of no switch connection

This patch improves operation when there is no switch connection.

Cc: Shivaram Mysore <shivaram.mysore@gmail.com>
Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>