Do not pass a raw pointer as the recursion key to rb_exec_recursive()

The spec added in 20ae94d passed a pointer to a 4-byte C stack struct
as both the `obj` and `arg` arguments of rb_exec_recursive(). While `arg`
is passed through to the callback as-is, `obj` is used as a key of the
recursion guard hash in CRuby. Storing it there fires the GC write
barrier, which reads the (8-byte) object flags from the non-object
address, and AddressSanitizer reports a stack-buffer-overflow:

    ==29649==ERROR: AddressSanitizer: stack-buffer-overflow ...
    READ of size 8 at 0x7f0d9b7c7b20 thread T0
        #0 RB_BUILTIN_TYPE include/ruby/internal/value_type.h:191:30
        #1 rb_gc_impl_writebarrier gc/default/default.c:6621:5
        ...
        #6 exec_recursive thread.c:5602:13
        #7 kernel_spec_rb_exec_recursive_with_pointer
           spec/ruby/optional/capi/ext/kernel_spec.c:370:10

This crash has been blocking the daily releases of
https://github.com/ruby/ruby-dev-builder (`build (ubuntu-24.04, asan)`)
since 2026-07-08.

The intent of the spec is that the callback argument is passed as-is,
so keep the raw pointer for `arg` and use a real Ruby object as the
recursion key.

Author: koic

optional/capi/ext/kernel_spec.c

@@ -352,9 +352,8 @@ struct rb_exec_recursive_pointer_data {
 };
 
 static VALUE do_rec_pointer(VALUE obj, VALUE arg, int is_rec) {
-  struct rb_exec_recursive_pointer_data *obj_data = (struct rb_exec_recursive_pointer_data *)obj;
   struct rb_exec_recursive_pointer_data *arg_data = (struct rb_exec_recursive_pointer_data *)arg;
-  if (obj_data->magic != 0x1234 || arg_data->magic != 0x1234) {
+  if (arg_data->magic != 0x1234) {
     rb_raise(rb_eRuntimeError, "invalid recursive pointer");
   }
 
@@ -367,7 +366,7 @@ static VALUE do_rec_pointer(VALUE obj, VALUE arg, int is_rec) {
 
 static VALUE kernel_spec_rb_exec_recursive_with_pointer(VALUE self) {
   struct rb_exec_recursive_pointer_data data = { 0x1234 };
-  return rb_exec_recursive(do_rec_pointer, (VALUE)&data, (VALUE)&data);
+  return rb_exec_recursive(do_rec_pointer, self, (VALUE)&data);
 }
 
 static void write_io(VALUE io) {