XHTTP client: Fix a race condition and a data race (#6665)

https://github.com/XTLS/Xray-core/pull/6665#issuecomment-5429028477

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Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

transport/internet/splithttp/dialer.go

@@ -595,11 +595,12 @@ func (w uploadWriter) Write(b []byte) (int, error) {
 
 	var writed int
 	for _, buff := range buffer.MultiBuffer {
+		n := int(buff.Len())
 		err := w.WriteMultiBuffer(buf.MultiBuffer{buff})
 		if err != nil {
 			return writed, err
 		}
-		writed += int(buff.Len())
+		writed += n
 	}
 	return writed, nil
 }