staging: comedi: fix result of memdup_user for user chanlist

If the channel list is not set in userspace we get an error at
PTR_ERR(async->cmd.chanlist). However, do_become_nonbusy(dev, s) cleans
up this pointer which causes a kernel ooops. Setting the channel list in
async to NULL and checking this in do_become_nonbusy prevents the oops.

[Ian Abbott] Also do the same for the chanlist allocated in
do_cmdtest_ioctl().
Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1473,6 +1473,7 @@ static int do_cmd_ioctl(struct comedi_device *dev,
 					  async->cmd.chanlist_len * sizeof(int));
 	if (IS_ERR(async->cmd.chanlist)) {
 		ret = PTR_ERR(async->cmd.chanlist);
+		async->cmd.chanlist = NULL;
 		dev_dbg(dev->class_dev, "memdup_user failed with code %d\n",
 			ret);
 		goto cleanup;
@@ -1595,6 +1596,7 @@ static int do_cmdtest_ioctl(struct comedi_device *dev,
 				       cmd.chanlist_len * sizeof(int));
 		if (IS_ERR(chanlist)) {
 			ret = PTR_ERR(chanlist);
+			chanlist = NULL;
 			dev_dbg(dev->class_dev,
 				"memdup_user exited with code %d", ret);
 			goto cleanup;