Fix race condition in allocating obfsBuf

internal/multiplex/stream.go

@@ -174,9 +174,11 @@ func (s *Stream) Write(in []byte) (n int, err error) {
 // ReadFrom continuously read data from r and send it off, until either r returns error or nothing has been read
 // for readFromTimeout amount of time
 func (s *Stream) ReadFrom(r io.Reader) (n int64, err error) {
+	s.writingM.Lock()
 	if s.obfsBuf == nil {
 		s.obfsBuf = make([]byte, s.session.StreamSendBufferSize)
 	}
+	s.writingM.Unlock()
 	for {
 		if s.readFromTimeout != 0 {
 			if rder, ok := r.(net.Conn); !ok {