sw test tool: Fix a problem for disconnection

Disconnection of switches has caused a RyuInternalError because target_sw/tester_sw is set to NoneType.
This patch fixes the problem.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>