Fix Buttons context, invalid object data access

Another instance of T44376.
Crash where the Python context would access a stale pointer to the active object.

source/blender/editors/include/ED_node.h

@@ -103,6 +103,8 @@ void ED_node_set_active(struct Main *bmain, struct bNodeTree *ntree, struct bNod
 
 void ED_node_composite_job(const struct bContext *C, struct bNodeTree *nodetree, struct Scene *scene_owner);
 
+void ED_node_id_unref(struct SpaceNode *snode, const ID *id);
+
 /* node_ops.c */
 void ED_operatormacros_node(void);