fix test_smtplib/test_smtpd collision through pre-loaded reply data in mock_socket
diff --git a/Lib/test/mock_socket.py b/Lib/test/mock_socket.py
index 1512642..8036932 100644
--- a/Lib/test/mock_socket.py
+++ b/Lib/test/mock_socket.py
@@ -36,6 +36,7 @@
         self.lines = []
         if _reply_data:
             self.lines.append(_reply_data)
+            _reply_data = None
         self.conn = None
         self.timeout = None