fix: Fix panic due to send on closed channel

```
2025-01-23T12:19:46+08:00 |INFO| New session: bf1be06db4b7c04cb951e5cb90e60f3b
panic: send on closed channel

goroutine 8 [running]:
main.(*user).WriteMsg(0xc000342140, 0x1, {0xc000124050, 0x41, 0x50})
        /mnt/wsl/work/rttys/user.go:52 +0x8b
main.userLoginAck(0x0, {0xf83100, 0xc000342140})
        /mnt/wsl/work/rttys/user.go:75 +0xc9
main.(*broker).run(0xc000270c80)
        /mnt/wsl/work/rttys/broker.go:212 +0xd91
created by main.runRttys in goroutine 1
        /mnt/wsl/work/rttys/main.go:86 +0x595
exit status 2
```

Run this script to reproduce:
```

local websocket = require 'eco.websocket'

local ws, err = websocket.connect('ws://127.0.0.1:5913/connect/test')
if not ws then
    print('failed to connect: ' .. err)
    return
end

os.exit()
```

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>

broker.go

@@ -138,6 +138,8 @@ func (br *broker) run() {
 				if dev, ok := br.devices[devid]; ok {
 					sid := utils.GenUniqueID("sid")
 
+					c.(*user).sid = sid
+
 					s := &session{
 						dev:  dev,
 						user: c,
@@ -206,9 +208,6 @@ func (br *broker) run() {
 				} else {
 					atomic.StoreUint32(&s.confirmed, 1)
 
-					u := s.user.(*user)
-					u.sid = msg.sid
-
 					userLoginAck(loginErrorNone, s.user)
 				}
 			}