Try to fix Leaked file descriptor

https://github.com/ruby/ruby/runs/2274767991?check_suite_focus=true#step:15:118
```
Leaked file descriptor: DRbTests::TestDRbTCP#test_immediate_close: 7 : #<TCPServer:fd 7, AF_INET, 0.0.0.0, 42451>
Leaked file descriptor: DRbTests::TestDRbTCP#test_immediate_close: 9 : #<IO:fd 9>
Leaked file descriptor: DRbTests::TestDRbTCP#test_immediate_close: 10 : #<IO:fd 10>
```

Author: znz

test/drb/test_drb.rb

@@ -342,7 +342,9 @@ def test_immediate_close
     socket.close
     client = DRb::DRbTCPSocket.new(server.uri, socket)
     assert client
-    client.close
+  ensure
+    client&.close
+    socket&.close
     server.stop_service
     server.thread.join
     DRb::DRbConn.stop_pool