obj_free_object_id: Don't fail if IMEMO/fields entry can't be found

In `build_id2ref_i` we ignore `T_IMEMO` objects, only consider the
object they old fields for. So it's possible that the owner object
has been garbage collected already, but not its IMEMO/fields.

Author: byroot

gc.c

@@ -2044,8 +2044,10 @@ obj_free_object_id(VALUE obj)
             RUBY_ASSERT(FIXNUM_P(obj_id) || RB_TYPE_P(obj_id, T_BIGNUM));
 
             if (!st_delete(id2ref_tbl, (st_data_t *)&obj_id, NULL)) {
-                // If we're currently building the table then it's not a bug
-                if (id2ref_tbl_built) {
+                // If we're currently building the table then it's not a bug.
+                // The the object is a T_IMEMO/fields, then it's possible the actual object
+                // has been garbage collected already.
+                if (id2ref_tbl_built && !RB_TYPE_P(obj, T_IMEMO)) {
                     rb_bug("Object ID seen, but not in _id2ref table: object_id=%llu object=%s", NUM2ULL(obj_id), rb_obj_info(obj));
                 }
             }