parse.y: invalid instance/class variable names

* parse.y (parse_atmark): mere atmark and two atmarks without
  succeeding identifiers are invalid as instance/class variable
  names.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48220 b2dd03c8-39d4-4d8f-98ff-823fe69b080e