tkutil.c: remove useless branches

* ext/tk/tkutil/tkutil.c (cbsubst_table_setup): remove useless
  branches and get rid of out of bound access.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e