Fix rb_free_generic_ivar to reset shape

Author: byroot

variable.c

@@ -1235,6 +1235,7 @@ rb_free_generic_ivar(VALUE obj)
 
         RB_VM_LOCKING() {
             st_delete(generic_fields_tbl_no_ractor_check(), &key, &value);
+            RBASIC_SET_SHAPE_ID(obj, ROOT_SHAPE_ID);
         }
     }
 }