parse.y: warn iside a block

* parse.y (gettable_gen): also warn circular argument reference even
  inside a block.  [ruby-core:65990] [Bug #10314]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48189 b2dd03c8-39d4-4d8f-98ff-823fe69b080e