staging: greybus: light: fix a couple double frees

The problem is in gb_lights_request_handler().  If we get a request to
change the config then we release the light with gb_lights_light_release()
and re-allocated it.  However, if the allocation fails part way through
then we call gb_lights_light_release() again.  This can lead to a couple
different double frees where we haven't cleared out the original values:

	gb_lights_light_v4l2_unregister(light);
	...
	kfree(light->channels);
	kfree(light->name);

I also made a small change to how we set "light->channels_count = 0;".
The original code handled this part fine and did not cause a use after
free but it was sort of complicated to read.

Fixes: 2870b52b ("greybus: lights: add lights implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Link: https://lore.kernel.org/r/20190829122839.GA20116@mwandaSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/light.c

@@ -1096,21 +1096,21 @@ static void gb_lights_channel_release(struct gb_channel *channel)
 static void gb_lights_light_release(struct gb_light *light)
 {
 	int i;
-	int count;
 
 	light->ready = false;
 
-	count = light->channels_count;
-
 	if (light->has_flash)
 		gb_lights_light_v4l2_unregister(light);
+	light->has_flash = false;
 
-	for (i = 0; i < count; i++) {
+	for (i = 0; i < light->channels_count; i++)
 		gb_lights_channel_release(&light->channels[i]);
-		light->channels_count--;
-	}
+	light->channels_count = 0;
+
 	kfree(light->channels);
+	light->channels = NULL;
 	kfree(light->name);
+	light->name = NULL;
 }
 
 static void gb_lights_release(struct gb_lights *glights)