tkutil.c: prevent temporary objects from GC

* ext/tk/tkutil/tkutil.c: use rb_sprintf(), rb_id2str(), and
  rb_intern_str() instead of rb_intern() and RSTRING_PTR() with
  RB_GC_GUARD(), to prevent temporary objects from GC.
  [ruby-core:39000] [Bug #5199]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41351 b2dd03c8-39d4-4d8f-98ff-823fe69b080e