Don't emit error in destructor of QSerialPort.

QSerialPort::~QSerialPort() calls close() which emits an error if the
port is already closed. This error signal may then call slots of
already (partially) deleted objects.

Task-number: QTBUG-39369
Change-Id: I28bf94564ae120619054c6920779da974dccdad6
Reviewed-by: Laszlo Papp <lpapp@kde.org>