报异常，然后挂掉： panic: runtime error: slice bounds out of range [69:66]

[ladixiu]

Describe the bug
使用UDP隧道，用上一会就会nps服务会挂掉，centos7系系统

错误堆栈
goroutine 49533 [running]:
ehang.io/nps-mux.(*sendWindow).WriteTo(0xc00045e000, 0x1000a2f803, 0xa54560, 0xc0006d3d58, 0x42, 0x0, 0x0)
/home/runner/go/pkg/mod/ehang.io/nps-mux@v0.0.0-20210407130203-4afa0c10c992/conn.go:554 +0x2f2
ehang.io/nps-mux.(*sendWindow).WriteFull(0xc00045e000, 0xc000a2f800, 0x42, 0x688, 0xc000000010, 0x40a37f, 0xc000010000, 0xaa0b00)
/home/runner/go/pkg/mod/ehang.io/nps-mux@v0.0.0-20210407130203-4afa0c10c992/conn.go:608 +0x8e
ehang.io/nps-mux.(*conn).Write(0xc00050ab40, 0xc000a2f800, 0x42, 0x688, 0xc000558e38, 0x40b905, 0xaa0b00)
/home/runner/go/pkg/mod/ehang.io/nps-mux@v0.0.0-20210407130203-4afa0c10c992/conn.go:63 +0x8e
ehang.io/nps/lib/rate.(*rateConn).Write(0xc000323140, 0xc000a2f800, 0x42, 0x688, 0xc000323140, 0x1, 0x0)
/home/runner/work/nps/nps/lib/rate/conn.go:28 +0x55
ehang.io/nps/server/proxy.(*UdpModeServer).process(0xc0004aa7e0, 0xc0003c94a0, 0xc000a2f800, 0x42, 0x688)
/home/runner/work/nps/nps/server/proxy/udp.go:59 +0x8e3
created by ehang.io/nps/server/proxy.(*UdpModeServer).Start
/home/runner/work/nps/nps/server/proxy/udp.go:50 +0x2cc
panic: runtime error: slice bounds out of range [69:66]