class.c: suppress wrong warning

* class.c (HAVE_METACLASS_P): should check FL_SINGLTON flag before get
  instance variable to get rid of wrong warning about __attached__.
  [ruby-core:53839] [Bug #8188]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40013 b2dd03c8-39d4-4d8f-98ff-823fe69b080e