media: saa7146: fix array overflow in vidioc_s_audio()

The "a->index" value comes from the user via the ioctl.  The problem is
that the shift can wrap resulting in setting "mxb->cur_audinput" to an
invalid value, which later results in an array overflow.

Fixes: 66804277 ("[media] mxb: fix audio handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/pci/saa7146/mxb.c

@@ -641,16 +641,17 @@ static int vidioc_s_audio(struct file *file, void *fh, const struct v4l2_audio *
 	struct mxb *mxb = (struct mxb *)dev->ext_priv;
 
 	DEB_D("VIDIOC_S_AUDIO %d\n", a->index);
-	if (mxb_inputs[mxb->cur_input].audioset & (1 << a->index)) {
-		if (mxb->cur_audinput != a->index) {
-			mxb->cur_audinput = a->index;
-			tea6420_route(mxb, a->index);
-			if (mxb->cur_audinput == 0)
-				mxb_update_audmode(mxb);
-		}
-		return 0;
+	if (a->index >= 32 ||
+	    !(mxb_inputs[mxb->cur_input].audioset & (1 << a->index)))
+		return -EINVAL;
+
+	if (mxb->cur_audinput != a->index) {
+		mxb->cur_audinput = a->index;
+		tea6420_route(mxb, a->index);
+		if (mxb->cur_audinput == 0)
+			mxb_update_audmode(mxb);
 	}
-	return -EINVAL;
+	return 0;
 }
 
 #ifdef CONFIG_VIDEO_ADV_DEBUG