fix: three high-severity bugs across nathole, proxy, and udp modules (#5214)

- pkg/nathole: add RLock when reading clientCfgs map in PreCheck path
  to prevent concurrent map read/write crash
- server/proxy: fix error variable shadowing in GetWorkConnFromPool
  that could return a closed connection with nil error
- pkg/util/net: check ListenUDP error before spawning goroutines
  and assign readConn to struct field so Close() works correctly

pkg/nathole/controller.go

@@ -152,7 +152,9 @@ func (c *Controller) GenSid() string {
 
 func (c *Controller) HandleVisitor(m *msg.NatHoleVisitor, transporter transport.MessageTransporter, visitorUser string) {
 	if m.PreCheck {
+		c.mu.RLock()
 		cfg, ok := c.clientCfgs[m.ProxyName]
+		c.mu.RUnlock()
 		if !ok {
 			_ = transporter.Send(c.GenNatHoleResponse(m.TransactionID, nil, fmt.Sprintf("xtcp server for [%s] doesn't exist", m.ProxyName)))
 			return