ext: use RARRAY_AREF
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52453 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
ext: use RARRAY_AREF
pointed out by hanmac.
https://github.com/ruby/ruby/commit/3553a86#commitcomment-14187670
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52453 b2dd03c8-39d4-4d8f-98ff-823fe69b080e