filter-buffer: fix segfault when starting qemu with status=off property

After commit 338d3f, we support 'status' property for filter object.
The segfault can be triggered by starting qemu with 'status=off' property
for filter, when the s->incoming_queue is NULL, we reference it directly
in qemu_net_queue_flush() which was called in status_changed() callback
function.

We shouldn't trigger status_changed() before the filter was initialized,
We can check the value of 'nf->netdev' to confirm if the filter is
initialized or not, so let's check its value before calling
status_changed().

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
