st-label: Unset clutter text instance on disposal

The instance is owned by the actor (being its child), and thus when the
disposal happens for the parent the text is disposed too, thus it's just
safer to nullify its reference so that we won't try to access to invalid
objects later, and this might be the case since the JS objects could be kept
around until they aren't finalized.

https://bugzilla.gnome.org/show_bug.cgi?id=788931


(cherry picked from commit 44894262f4)

src/st/st-label.c

@@ -180,6 +180,7 @@ st_label_dispose (GObject   *object)
 {
   StLabelPrivate *priv = ST_LABEL (object)->priv;
 
+  priv->label = NULL;
   g_clear_pointer (&priv->text_shadow_pipeline, cogl_object_unref);
 
   G_OBJECT_CLASS (st_label_parent_class)->dispose (object);