d/uvc: Mark stream as not running before locking event waiting

Fixes a race condition on exit

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2750>

src/xrt/drivers/uvc/uvc_driver.c

@@ -381,8 +381,8 @@ uvc_fs_stream_stop(struct xrt_fs *xfs)
 		         strerror(errno));
 	}
 
-	libusb_lock_event_waiters(stream->usb_ctx);
 	stream->is_running = false;
+	libusb_lock_event_waiters(stream->usb_ctx);
 
 	// Wait for active transfers to finish
 	while (stream->active_transfers > 0) {