`(attr_index + 1)` leads to wrong integer expansion on 32-bit machines
(including Solaris 10 CI) because `attr_index_t` is uint16_t.

http://rubyci.s3.amazonaws.com/solaris10-gcc/ruby-master/log/20221013T080004Z.fail.html.gz
```
  1) Failure:
TestRDocClassModule#test_marshal_load_version_2 [/export/home/users/chkbuild/cb-gcc/tmp/build/20221013T080004Z/ruby/test/rdoc/test_rdoc_class_module.rb:493]:
<[doc: [doc (file.rb): [para: "this is a comment"]]]> expected but was
<[doc: [doc (file.rb): [para: "this is a comment"]]]>.

  2) Failure:
TestRDocStats#test_report_method_line [/export/home/users/chkbuild/cb-gcc/tmp/build/20221013T080004Z/ruby/test/rdoc/test_rdoc_stats.rb:460]:
Expected /\#\ in\ file\ file\.rb:4/ to match "The following items are not documented:\n" +
"\n" +
"  class C # is documented\n" +
"\n" +
"    # in file file.rb\n" +
"    def m1; end\n" +
"\n" +
"  end\n" +
"\n" +
"\n".

  3) Failure:
TestRDocStats#test_report_attr_line [/export/home/users/chkbuild/cb-gcc/tmp/build/20221013T080004Z/ruby/test/rdoc/test_rdoc_stats.rb:91]:
Expected /\#\ in\ file\ file\.rb:3/ to match "The following items are not documented:\n" +
"\n" +
"  class C # is documented\n" +
"\n" +
"    attr_accessor :a # in file file.rb\n" +
"\n" +
"  end\n" +
"\n" +
"\n".
```