parse.y: pin down dynamic symbol only

* parse.y (rb_id_attrset): pin down dynamic symbol only. it is
  possibe that attrset ID can be registered as a static symbol
  after the corresponding attrget ID has been registered as a
  dynamic, and then the latter may be collected.
  [ruby-core:62226] [Bug #9787]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45756 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-30

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45757 b2dd03c8-39d4-4d8f-98ff-823fe69b080e