id_table.c: use NULL as ID* instead of Qundef
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67495 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
id_table.c: use NULL as ID* instead of Qundef
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67495 b2dd03c8-39d4-4d8f-98ff-823fe69b080e