Initializing the variable 's' so that the get_desc_stats() call does not cause an exceptions when there are no messages

Signed-off-by: Srini Seetharaman <srini.seetharaman@gmail.com>
Reviewed-by: TAKAHASHI Minoru <takahashi.minoru7@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>