virtio/virtio_mem: handle a possible NULL as a memcpy parameter

There is a check for vm->sbm.sb_states before, and it should check
it here as well.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Link: https://lore.kernel.org/r/20211222011225.40573-1-flyingpeng@tencent.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: 5f1f79bb ("virtio-mem: Paravirtualized memory hotplug")
Cc: stable@vger.kernel.org # v5.8+