... because GCC 13 warns it.

```
In file included from class.c:24:
In function ‘RCLASS_SET_ALLOCATOR’,
    inlined from ‘class_alloc’ at class.c:251:5,
    inlined from ‘rb_module_s_alloc’ at class.c:1045:17:
internal/class.h:159:43: warning: array subscript 0 is outside array bounds of ‘rb_classext_t[0]’ {aka ‘struct rb_classext_struct[]’} [-Warray-bounds=]
  159 |     RCLASS_EXT(klass)->as.class.allocator = allocator;
      |                                           ^
```
https://rubyci.s3.amazonaws.com/arch/ruby-master/log/20231015T030003Z.log.html.gz