usb: gadget: function: uvc: conditionally dequeue

We shouldn't try to dequeue a NULL pointer.
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/uvc_video.c

@@ -352,6 +352,7 @@ int uvcg_video_enable(struct uvc_video *video, int enable)
 
 	if (!enable) {
 		for (i = 0; i < UVC_NUM_REQUESTS; ++i)
+			if (video->req[i])
 				usb_ep_dequeue(video->ep, video->req[i]);
 
 		uvc_video_free_requests(video);