igmplib: Fix a problem that querier stops sending a query message

When QUERY timeout expires, a variable for flooding is overwritten unexpectedly.
So querier stops sending a query message any longer.
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>