ClutterDragAction: do not dereference a NULL priv->stage
ClutterDragAction: do not dereference a NULL priv->stage

When destroying an actor during a drag-action with a drag_handle, the
ClutterDragAction's priv->stage could be set to NULL in _set_actor().
_dispose() must not assume that priv->stage is not NULL.

See also https://bugzilla.gnome.org/show_bug.cgi?id=681814

djdeath/clutter,djdeath/clutter,collects/clutter,GNOME/clutter,collects/clutter,ebassi/clutter,collects/clutter,jigpu/clutter,collects/clutter,GNOME/clutter,Distrotech/clutter,ebassi/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,heysion/clutter-clone,jigpu/clutter,djdeath/clutter,Distrotech/clutter,Distrotech/clutter,heysion/clutter-clone,GNOME/clutter,ebassi/clutter,GNOME/clutter,jigpu/clutter,heysion/clutter-clone,GNOME/clutter,djdeath/clutter,collects/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone