parse.y: warn reference after method definition

* parse.y (primary): restore current_arg so that circular
  reference after a method definition is also warned.
  [ruby-core:61299] [Bug #9593]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48835 b2dd03c8-39d4-4d8f-98ff-823fe69b080e