[Bug] panic: send on closed channel

[Aruelius]

Describe the bug【描述 bug】

panic: send on closed channel

goroutine 94388 [running]:
github.com/e1732a364fed/v2ray_simple/netLayer.(*UDPMsgConn).readSymmetricMsgFromConn(0xc000287100, 0xc000012198, {{0xe5a0ff, 0x3}, {0x0, 0x0}, {{{0x0, 0xffff9df00d13}, 0xc0000bc0a8}, 0x1bb}})
        github.com/e1732a364fed/v2ray_simple/netLayer/msgconn.go:163 +0x2a5
created by github.com/e1732a364fed/v2ray_simple/netLayer.(*UDPMsgConn).WriteMsg
        github.com/e1732a364fed/v2ray_simple/netLayer/msgconn.go:219 +0x615

To Reproduce【如何复现该bug】
v1.2.5 版本从 25 号到现在崩溃两次，进程自动退出

Envs (please complete the following information):【系统环境】

Linux

[Aruelius]

代码中判断一下准备写入的通道状态可以解决，这个问题是在客户端用 v2rayNG 时候出现的，和设置的远程 DNS 有关，应该是 v2rayNG 会去请求一次该 DNS 导致的：

"Target": "udp://1.1.1.1:53"

[e1732a364fed]

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