tkutil.c: fix out-of-bounds access
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50708 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
tkutil.c: fix out-of-bounds access
length not access out-of-bounds.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50708 b2dd03c8-39d4-4d8f-98ff-823fe69b080e