Fix instance variable not defined warning from Active Support test suite

Before

./Users/Juan/dev/rails/activesupport/test/file_update_checker_shared_tests.rb:20: warning: instance variable @tmpdir not initialized

After

No warnings

activesupport/test/file_update_checker_shared_tests.rb

@@ -17,7 +17,7 @@ def tmpfiles
   end
 
   def teardown
-    FileUtils.rm_rf(@tmpdir) if @tmpdir
+    FileUtils.rm_rf(@tmpdir) if defined? @tmpdir
   end
 
   test 'should not execute the block if no paths are given' do