rocker: fix memory leak

Meanwhile, using g_new0 instead of g_malloc0,
refer to commit 5839e53.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-id: 1435213450-6700-1-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
