Panic when closing a connection

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Close a client connection and fetch the ClientAddr() in OnConnClose().

2. What did you expect to see? (Required)

The address is right.

3. What did you see instead (Required)

Panic.
Because the PacketIO.conn is set to nil after closing.

4. What is your version? (Required)

master