V4L/DVB (12844): tm6000: avoid writing on a floating pointer

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/staging/tm6000/tm6000-video.c

@@ -139,6 +139,7 @@ static inline void get_next_buf(struct tm6000_dmaqueue *dma_q,
 
 	if (list_empty(&dma_q->active)) {
 		dprintk(dev, V4L2_DEBUG_QUEUE, "No active queue to serve\n");
+		*buf = NULL;
 		return;
 	}