[REG 5.15 -> 6.0.0] Crash when using iterators to default constructed QString

void tst_QString::std_mismatch()
{
    const QString s1("blabla");
    const QString s2;
    const auto mismatches = std::mismatch(s1.begin(),
                                          s1.end(),
                                          s2.begin());
    const int commonPrefixCount = mismatches.first - s1.begin();
    QCOMPARE(commonPrefixCount, 0);
}