staging: comedi: ni_tio: make ni_gpct_device_destroy() NULL pointer safe

Modify the pointer check to make this function NULL pointer safe.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_tio.c

@@ -1413,7 +1413,7 @@ EXPORT_SYMBOL_GPL(ni_gpct_device_construct);
 
 void ni_gpct_device_destroy(struct ni_gpct_device *counter_dev)
 {
-	if (!counter_dev->counters)
+	if (!counter_dev)
 		return;
 	kfree(counter_dev->counters);
 	kfree(counter_dev);