libweston: move asserts to add_pending_output()

weston_compositor_add_pending_output() is the point through which all
backends must go when creating a new output. The enable and disable
vfuns are essential for anything to be done with the output, so it makes
sense to check them here, rather than when actually enabling or
disabling.

Particularly the disable vfunc is rarely called, so this gets the check
better excercised.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>

libweston/compositor.c

@@ -4701,6 +4701,9 @@ WL_EXPORT void
 weston_compositor_add_pending_output(struct weston_output *output,
 				     struct weston_compositor *compositor)
 {
+	assert(output->disable);
+	assert(output->enable);
+
 	wl_list_remove(&output->link);
 	wl_list_insert(compositor->pending_output_list.prev, &output->link);
 	wl_signal_emit(&compositor->output_pending_signal, output);
@@ -4745,8 +4748,6 @@ weston_output_enable(struct weston_output *output)
 	struct weston_output *iterator;
 	int x = 0, y = 0;
 
-	assert(output->enable);
-
 	iterator = container_of(c->output_list.prev,
 				struct weston_output, link);
 
@@ -4833,8 +4834,6 @@ weston_output_enable(struct weston_output *output)
 WL_EXPORT void
 weston_output_disable(struct weston_output *output)
 {
-	assert(output->disable);
-
 	/* Should we rename this? */
 	output->destroying = 1;