Checkmk Werk 7303: smart: Handle snmp data without normalized values correctly

ID: 7303 Title: smart: Handle snmp data without normalized values correctly Component: Checks & agents Level: 1 Class: Bug fix Version: 1.6.0i1 SNMP data may contain strings "---" for the OIDs which represent the normalized value and threshold. For the "Reallocated Event Count" missing valid data resulted in a crash of the check function. This werk fixes this by adding proper handling of this edge case.