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

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.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>