controller: fix that sender thread is never terminated

Empty q.get() is blocking send thread.
Because of this issue, datapath state never transit to DEAD.

This fixes commit 83e3709a0d70f889e794d1c5f70c1c1eb73075f7.
  - controller: fix send_q draining
      https://github.com/osrg/ryu/commit/83e3709a0d70f889e794d1c5f70c1c1eb73075f7

Signed-off-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>