gamma-control-v1: fix handling of duplicate control

When a new client comes in and tries to create a
zwlr_gamma_control_v1 object for an output which already has one,
we were destroying the old object and early-returning. The early
return causes the new object to not be set up properly: it's not
inserted in the list and doesn't get the gamma_size event.

Fix this by destroying the new object, and leave the old object
intact, which is what was the intent I think.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3605

types/wlr_gamma_control_v1.c

@@ -219,8 +219,8 @@ static void gamma_control_manager_get_gamma_control(struct wl_client *client,
 	struct wlr_gamma_control_v1 *gc;
 	wl_list_for_each(gc, &manager->controls, link) {
 		if (gc->output == output) {
-			zwlr_gamma_control_v1_send_failed(gc->resource);
-			gamma_control_destroy(gc);
+			zwlr_gamma_control_v1_send_failed(gamma_control->resource);
+			gamma_control_destroy(gamma_control);
 			return;
 		}
 	}