[mutter] compositor: Fix maybe-uninitialized warning

[Jonas Ådahl <jadahl src gnome org>]

commit e7fb45364a64934484302e38b1f306f1a86b71e4
Author: Robert Mader <robert mader posteo de>
Date:   Mon Jan 7 12:35:29 2019 +0100

    compositor: Fix maybe-uninitialized warning
    
    Introduced in 54febd1419e927f6e3294185e0a595439f32becf
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/374

 src/compositor/compositor.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index f031bd151..0854a198b 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -665,7 +665,7 @@ meta_compositor_add_window (MetaCompositor    *compositor,
   MetaWindowActor *window_actor;
   ClutterActor *window_group;
   MetaDisplay *display = compositor->display;
-  GType window_actor_type;
+  GType window_actor_type = G_TYPE_INVALID;
 
   meta_x11_error_trap_push (display->x11_display);