Sometimes path nodes can survive longer than the style context that
created them. Don't crash in those cases.
Fixes startup of mutter.
Testcase included.
https://bugzilla.gnome.org/show_bug.cgi?id=746407
- GtkCssWidgetNode
for style contexts owned by a widget
- GtkCssPathNode
for style contexts using a GtkWidgetPath
- GtkCssTransientNode
for nodes created with gtk_style_context_save()/restore()