ALSA: usx2y: Fix potential NULL dereference

The error handling code in usX2Y_rate_set() may hit a potential NULL
dereference when an error occurs before allocating all us->urb[].
Add a proper NULL check for fixing the corner case.
Reported-by: Lin Yi <teroincn@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200420075529.27203-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usx2y/usbusx2yaudio.c

@@ -681,6 +681,8 @@ static int usX2Y_rate_set(struct usX2Ydev *usX2Y, int rate)
 			us->submitted =	2*NOOF_SETRATE_URBS;
 			for (i = 0; i < NOOF_SETRATE_URBS; ++i) {
 				struct urb *urb = us->urb[i];
+				if (!urb)
+					continue;
 				if (urb->status) {
 					if (!err)
 						err = -ENODEV;