background: don't tank if background is destroyed before it gets a pipeline

Right now we call unset_texture from MetaBackground's dispose method.

unset_texture assumes there's a pipeline available, but there may not
be if the object was just created.

This commit fixes that incorrect assumption.

https://bugzilla.gnome.org/show_bug.cgi?id=696157

src/compositor/meta-background.c

@@ -704,7 +704,8 @@ static void
 unset_texture (MetaBackground *self)
 {
   MetaBackgroundPrivate *priv = self->priv;
-  cogl_pipeline_set_layer_texture (priv->pipeline, 0, NULL);
+  if (priv->pipeline != NULL)
+    cogl_pipeline_set_layer_texture (priv->pipeline, 0, NULL);
 
   g_clear_pointer (&priv->texture,
                    (GDestroyNotify)