Don't call f.close if f is nil or already closed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35229 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/open-uri/test_open-uri.rb

@@ -152,7 +152,7 @@ class TestOpenURI < Test::Unit::TestCase
         assert_equal("200", f.status[0])
         assert_equal("without_block", f.read)
       ensure
-        f.close
+        f.close if f && !f.closed?
       end
     }
   end