drivers/usb/misc: Use an IS_ERR test rather than a NULL test

From: Julien Brunel <[email protected]>

In case of error, the function backlight_device_register returns an ERR pointer, but never returns a NULL pointer. So a NULL test that may come after a call to this function should be strengthened by an IS_ERR test.

The semantic match that finds this problem is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@match_bad_null_test@
expression x, E;
statement S1,S2;
@@
x = backlight_device_register(...)
... when != x = E
* if (x != NULL)
S1 else S2
// </smpl>

---
 drivers/usb/misc/appledisplay.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -u -p a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
@@ -314,7 +314,7 @@ error:
 					pdata->urbdata, pdata->urb->transfer_dma);
 			usb_free_urb(pdata->urb);
 		}
-		if (pdata->bd)
+		if (pdata->bd && !IS_ERR(pdata->bd))
 			backlight_device_unregister(pdata->bd);
 		kfree(pdata->msgdata);
 	}