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.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: 5f1f79bb ("virtio-mem: Paravirtualized memory hotplug")
Cc: stable@vger.kernel.org # v5.8+

drivers/virtio/virtio_mem.c

@@ -593,7 +593,7 @@ static int virtio_mem_sbm_sb_states_prepare_next_mb(struct virtio_mem *vm)
 		return -ENOMEM;
 
 	mutex_lock(&vm->hotplug_mutex);
-	if (new_bitmap)
+	if (vm->sbm.sb_states)
 		memcpy(new_bitmap, vm->sbm.sb_states, old_pages * PAGE_SIZE);
 
 	old_bitmap = vm->sbm.sb_states;