Crash when QGb18030Codec::convertFromUnicode converts surrogates

// code placeholder
QByteArray QGb18030Codec::convertFromUnicode(const QChar *uc, int len, ConverterState *state) const
{
    ...
    //qDebug("QGb18030Codec::fromUnicode(const QString& uc, int& lenInOut = %d)", lenInOut);
    for (int i = 0; i < len; i++) {
        unsigned short ch = uc[i].unicode();
        int len;
        uchar buf[4];
        if (high >= 0) {
            if (uc[i].isLowSurrogate()) {
                // valid surrogate pair
                ++i;
                uint u = QChar::surrogateToUcs4(high, uc[i].unicode());
                len = qt_UnicodeToGb18030(u, buf);
    ...

++i;