Fix T37939: Crash on exit (reading from freed node trees)

source/blender/blenkernel/intern/library.c

@@ -950,7 +950,7 @@ void BKE_libblock_free_ex(ListBase *lb, void *idv, bool do_id_user)
 			BKE_action_free((bAction *)id);
 			break;
 		case ID_NT:
-			ntreeFreeTree((bNodeTree *)id);
+			ntreeFreeTree_ex((bNodeTree *)id, do_id_user);
 			break;
 		case ID_BR:
 			BKE_brush_free((Brush *)id);

source/blender/blenkernel/intern/node.c

@@ -1714,8 +1714,11 @@ void ntreeFreeTree_ex(bNodeTree *ntree, const short do_id_user)
 		}
 	}
 	
+	/* when freeing main, this would check other ntree's which may have been freed, see [#37939] */
+	if (do_id_user) {
 		/* XXX not nice, but needed to free localized node groups properly */
 		free_localized_node_groups(ntree);
+	}
 	
 	/* unregister associated RNA types */
 	ntreeInterfaceTypeFree(ntree);