fix: Fix panic: send on closed channel

```
device                           broker
------------------------------------------------------
dev.br.register <- dev

dev.br.unregister <- dev
                                 handle unregister
                                       c.Close()

                                 handle register
                                       c.WriteMsg(msgTypeRegister, append([]byte{err}, msg...))
                                            dev.send <- append(b, data...)

```

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

broker.go

@@ -66,6 +66,10 @@ func (br *broker) run() {
 	for {
 		select {
 		case c := <-br.register:
+			if c.Closed() {
+				break
+			}
+
 			devid := c.DeviceID()
 
 			if c.IsDevice() {

client/client.go

@@ -11,4 +11,6 @@ type Client interface {
 	IsDevice() bool
 
 	Close()
+
+	Closed() bool
 }

device.go

@@ -131,10 +131,15 @@ func (dev *device) WriteMsg(typ int, data []byte) {
 	dev.send <- append(b, data...)
 }
 
+func (dev *device) Closed() bool {
+	return atomic.LoadUint32(&dev.closed) == 1
+}
+
 func (dev *device) Close() {
-	if atomic.LoadUint32(&dev.closed) == 1 {
+	if dev.Closed() {
 		return
 	}
+
 	atomic.StoreUint32(&dev.closed, 1)
 
 	log.Debug().Msgf("Device '%s' disconnected", dev.conn.RemoteAddr())

user.go

@@ -55,10 +55,15 @@ func (u *user) WriteMsg(typ int, data []byte) {
 	}
 }
 
+func (u *user) Closed() bool {
+	return atomic.LoadUint32(&u.closed) == 1
+}
+
 func (u *user) Close() {
-	if atomic.LoadUint32(&u.closed) == 1 {
+	if u.Closed() {
 		return
 	}
+
 	atomic.StoreUint32(&u.closed, 1)
 
 	u.conn.Close()