V4L/DVB: v4l: videobuf: prevent passing a NULL to dma_free_coherent()

When a driver that uses videobuf-dma-contig is used with the USERPTR
memory access method a kernel oops might happen: a NULL address may be
passed to dma_free_coherent(). This happens when an application calls
REQBUFS and then exits without queuing any buffers. This patch fixes
that bug.
Signed-off-by: Pawel Osciak <p.osciak@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/videobuf-dma-contig.c

@@ -393,8 +393,10 @@ void videobuf_dma_contig_free(struct videobuf_queue *q,
 	}
 
 	/* read() method */
-	dma_free_coherent(q->dev, mem->size, mem->vaddr, mem->dma_handle);
-	mem->vaddr = NULL;
+	if (mem->vaddr) {
+		dma_free_coherent(q->dev, mem->size, mem->vaddr, mem->dma_handle);
+		mem->vaddr = NULL;
+	}
 }
 EXPORT_SYMBOL_GPL(videobuf_dma_contig_free);