[media] ivtv: steal could be NULL

ivtv_flush_queues() calls ivtv_queue_move() with steal == NULL.
However, part of the code assumes that steal could be not null, as
pointed by smatch:
	drivers/media/pci/ivtv/ivtv-queue.c:145 ivtv_queue_move() error: we previously assumed 'steal' could be null (see line 138)

This has the potential of causing an OOPS when the queue is
flushed.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/pci/ivtv/ivtv-queue.c

@@ -141,7 +141,7 @@ int ivtv_queue_move(struct ivtv_stream *s, struct ivtv_queue *from, struct ivtv_
 		spin_unlock_irqrestore(&s->qlock, flags);
 		return -ENOMEM;
 	}
-	while (bytes_available < needed_bytes) {
+	while (steal && bytes_available < needed_bytes) {
 		struct ivtv_buffer *buf = list_entry(steal->list.prev, struct ivtv_buffer, list);
 		u16 dma_xfer_cnt = buf->dma_xfer_cnt;