usb: gadget: uvc: test if ep->desc is valid on ep_queue

The reason that the ep_queue has failed could be a disabled endpoint.
In that case it is not guaranteed that the ep->desc is still valid.
This patch adds a check for NULL.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20211017215017.18392-4-m.grzeschik@pengutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/uvc_video.c

@@ -199,9 +199,12 @@ static int uvcg_video_ep_queue(struct uvc_video *video, struct usb_request *req)
 		uvcg_err(&video->uvc->func, "Failed to queue request (%d).\n",
 			 ret);
 
-		/* Isochronous endpoints can't be halted. */
-		if (usb_endpoint_xfer_bulk(video->ep->desc))
-			usb_ep_set_halt(video->ep);
+		/* If the endpoint is disabled the descriptor may be NULL. */
+		if (video->ep->desc) {
+			/* Isochronous endpoints can't be halted. */
+			if (usb_endpoint_xfer_bulk(video->ep->desc))
+				usb_ep_set_halt(video->ep);
+		}
 	}
 
 	return ret;