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/util/net/udp.go

@@ -168,11 +168,15 @@ func ListenUDP(bindAddr string, bindPort int) (l *UDPListener, err error) {
 		return l, err
 	}
 	readConn, err := net.ListenUDP("udp", udpAddr)
+	if err != nil {
+		return l, err
+	}
 
 	l = &UDPListener{
 		addr:      udpAddr,
 		acceptCh:  make(chan net.Conn),
 		writeCh:   make(chan *UDPPacket, 1000),
+		readConn:  readConn,
 		fakeConns: make(map[string]*FakeUDPConn),
 	}