Fix: Header crc calculation

Signed-off-by: Thomas Klaehn <tkl@blackfinn.de>

fitparser/header.go

@@ -54,12 +54,14 @@ func ParseHeader(data []byte) (*Header, error) {
 	if h.Size == HeaderWithCRCSize {
 		h.CRC = binary.LittleEndian.Uint16(data[12:14])
 
-		// Validate header CRC
+		// Validate header CRC (skip if CRC is 0x0000, which means "no validation")
+		if h.CRC != 0x0000 {
 			calculatedCRC := CalculateCRC(data[0:12])
 			if calculatedCRC != h.CRC {
 				return nil, fmt.Errorf("header CRC mismatch: got 0x%04X, expected 0x%04X", h.CRC, calculatedCRC)
 			}
 		}
+	}
 
 	return h, nil
 }