st: Don't try to create shadow for empty texture

Even though the API documentation doesn't say so, the underlying
Cogl texture of a ClutterTexture may be unset, so check for that
case to avoid a runtime warning.

https://bugzilla.gnome.org/show_bug.cgi?id=784353

src/st/st-private.c

@@ -420,6 +420,7 @@ _st_create_shadow_pipeline_from_actor (StShadow     *shadow_spec,
       CoglTexture *texture;
 
       texture = clutter_texture_get_cogl_texture (CLUTTER_TEXTURE (actor));
+      if (texture)
         shadow_pipeline = _st_create_shadow_pipeline (shadow_spec, texture);
     }
   else