Handle non-enum values to fix -Wreturn-type

http://ci.rvm.jp/results/trunk-random1@ruby-sp2-docker/4612360

variable.c

@@ -1752,6 +1752,7 @@ iterate_over_shapes_with_callback(rb_shape_t *shape, rb_ivar_foreach_callback_fu
       case SHAPE_T_OBJECT:
         return iterate_over_shapes_with_callback(rb_shape_get_parent(shape), callback, itr_data);
       case SHAPE_OBJ_TOO_COMPLEX:
+      default:
         rb_bug("Unreachable");
     }
 }