symbol.c: fix dynamic attrset ID

* symbol.c (rb_str_dynamic_intern): check if the stem ID of
  attrset ID is already registered as a static ID.
  [ruby-dev:48559] [Bug #10259]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47643 b2dd03c8-39d4-4d8f-98ff-823fe69b080e