Fix crash /test/guiapp upon receipt of the index (-1) in the list of ports (if not match any port).
Fix crash /test/guiapp upon receipt of the index (-1) in the list of ports (if not match any port).

kobolabs/qtserialport,kobolabs/qtserialport,RoadWorksSoftware/QtSerialPort-OS2,RoadWorksSoftware/QtSerialPort-OS2