XHTTP client: Fix edge-case issue for packet-up mode (#5020)

https://github.com/XTLS/Xray-core/pull/4952#issuecomment-3184080580

transport/internet/splithttp/dialer.go

@@ -489,15 +489,16 @@ func (w uploadWriter) Write(b []byte) (int, error) {
 		}
 	*/
 
-	buffer := buf.New()
-	n, err := buffer.Write(b)
-	if err != nil {
-		return 0, err
-	}
+	buffer := buf.MultiBufferContainer{}
+	common.Must2(buffer.Write(b))
 
-	err = w.WriteMultiBuffer([]*buf.Buffer{buffer})
-	if err != nil {
-		return 0, err
+	var writed int
+	for _, buff := range buffer.MultiBuffer {
+		err := w.WriteMultiBuffer(buf.MultiBuffer{buff})
+		if err != nil {
+			return writed, err
+		}
+		writed += int(buff.Len())
 	}
-	return n, nil
+	return writed, nil
 }