Mux client: Fix b.UDP being overwritten by the next frame (#6822)

https://github.com/XTLS/Xray-core/pull/6822#issuecomment-5993165787

common/mux/reader.go

@@ -14,15 +14,16 @@ import (
 type PacketReader struct {
 	reader io.Reader
 	eof    bool
-	dest   *net.Destination
+	dest   net.Destination
 }
 
 // NewPacketReader creates a new PacketReader.
+// dest is copied because the caller reuses it for the next frame.
 func NewPacketReader(reader io.Reader, dest *net.Destination) *PacketReader {
 	return &PacketReader{
 		reader: reader,
 		eof:    false,
-		dest:   dest,
+		dest:   *dest,
 	}
 }
 
@@ -47,8 +48,8 @@ func (r *PacketReader) ReadMultiBuffer() (buf.MultiBuffer, error) {
 		return nil, err
 	}
 	r.eof = true
-	if r.dest != nil && r.dest.Network == net.Network_UDP {
-		b.UDP = r.dest
+	if r.dest.Network == net.Network_UDP {
+		b.UDP = &r.dest // only one packet is read, so b owns r.dest
 	}
 	return buf.MultiBuffer{b}, nil
 }