tkutil.c: fix memory leak and segfault

* ext/tk/tkutil/tkutil.c (cbsubst_append_inf_key): extract a
  function append a key in subst info to a string.  make result
  strings first and get rid of potential memory leak.

* ext/tk/tkutil/tkutil.c (cbsubst_get_subst_arg): allocate the
  result buffer as a string to fix:

  * memory leak when the argument key is not found:

      loop {Tk::Event.subst_arg(:a) rescue nil}

  * buffer overflow segfault when many arguments:

      class T < TkUtil::CallbackSubst
        _setup_subst_table([[?a, ?A, :_a]], [])
        subst_arg(*[:_a]*1000).size
      end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50704 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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