media: tw5864: Fix possible NULL pointer dereference in tw5864_handle_frame

'vb' null check should be done before dereferencing it in
tw5864_handle_frame, otherwise a NULL pointer dereference
may occur.

Fixes: 34d1324e ("[media] pci: Add tw5864 driver")
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/pci/tw5864/tw5864-video.c

@@ -1395,13 +1395,13 @@ static void tw5864_handle_frame(struct tw5864_h264_frame *frame)
 	input->vb = NULL;
 	spin_unlock_irqrestore(&input->slock, flags);
 
-	v4l2_buf = to_vb2_v4l2_buffer(&vb->vb.vb2_buf);
-
 	if (!vb) { /* Gone because of disabling */
 		dev_dbg(&dev->pci->dev, "vb is empty, dropping frame\n");
 		return;
 	}
 
+	v4l2_buf = to_vb2_v4l2_buffer(&vb->vb.vb2_buf);
+
 	/*
 	 * Check for space.
 	 * Mind the overhead of startcode emulation prevention.