Windows: QTextStream::readLine crashes when QTextCodec::codecForLocale() returns NULL

This is similar to QTBUG-38496. Programs crash when trying to use QTextStream::readLine, Qt was built without QT_NO_TEXTCODEC, and no proper codecs are found (as seems to happen when building with (some versions of) icu). A minimal test case (assuming a file named "test.txt" exists):

#include <QtGui>
#include <iostream>
int main(int argc, char *argv[])
{
        QFile fin("test.txt");
        if (!fin.open(QIODevice::ReadOnly | QIODevice::Text))
                return 1;
        QTextStream ts(&fin);
        std::cout << qPrintable(ts.readLine());
        return 0;
}

In QTextStreamPrivate::fillReadBuffer(), if no codec is found, it is set to QTextCodec::codecForLocale() (line 451), which may return NULL. On line 472, this is used in codec->toUnicode(...) without checking if codec is NULL, thus crashing the application. I suspect there are other places where the same might happen, but I haven't investigated.

For a test program to check which codecs are found, see QTBUG-38496.