test_io.rb: fix assertion
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_io.rb: fix assertion
by assert_nil.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e