shadow: Don't attempt to create a shadow for unallocated actors

Calling clutter_actor_get_allocation_box() can potentially cause a
re-layout, which in _st_create_shadow_pipeline_from_actor() would
mean invalidating the shadow_spec passed as a parameter, which
could crash the shell later on since the value for shadow_spec->blur
passed from _st_create_shadow_pipeline() to blur_pixels() would
not be a valid one.

Thus, whether the ClutterActor has an allocation before calling
clutter_actor_get_allocation_box(), and early return if needed.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/36

Closes: #36

src/st/st-private.c

@@ -417,6 +417,11 @@ _st_create_shadow_pipeline_from_actor (StShadow     *shadow_spec,
   ClutterActorBox box;
   float width, height;
 
+  /* Calling clutter_actor_get_allocation_box() can provoke a re-layout, which
+   * would invalidate the shadow_spec, so check this and early return if needed. */
+  if (!clutter_actor_has_allocation(actor))
+    return NULL;
+
   clutter_actor_get_allocation_box (actor, &box);
   clutter_actor_box_get_size (&box, &width, &height);