[ruby/rdoc] Check uninitialized instance variable in test

https://github.com/ruby/rdoc/commit/3dcd5ddbb6

test/rdoc/support/test_case.rb

@@ -76,7 +76,7 @@ class RDoc::TestCase < Test::Unit::TestCase
   # Abstract test-case teardown
 
   def teardown
-    ENV["HOME"] = @orig_home
+    ENV["HOME"] = @orig_home if defined?(@orig_home)
 
     super
   end