[REG 6.0.4 -> 6.1.0] Integer-overflow in QFixed::operator+= through QImage::loadFromData(QByteArray)

1. Configure Qt with "-sanitize fuzzer-no-link -sanitize undefined" and build it.
   Having submodules qtbase and qtsvg is sufficient.
2. Use this to build the project qtbase/tests/libfuzzer/gui/image/qimage/loadfromdata.
   qmake- and cmake-based version lead to the same result.
3. Run the resulting program passing the attached input file:

   ./loadfromdata 36218.svg
   

   You'll get output containing lines like:

   /home/qtrob/dev/src/qt-dev_07.13-base_svg/qtbase/src/gui/painting/qpaintengine_raster.cpp:538:40: runtime error: -1,84467e+19 is outside the range of representable values of type 'int'
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/qtrob/dev/src/qt-dev_07.13-base_svg/qtbase/src/gui/painting/qpaintengine_raster.cpp:538:40 in 
   qtbase/include/QtGui/6.2.0/QtGui/private/../../../../../../../../src/qt-dev_07.13-base_svg/qtbase/src/gui/painting/qfixed_p.h:90:58: runtime error: signed integer overflow: 2147169024 + 655424 cannot be represented in type 'int'
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior qtbase/include/QtGui/6.2.0/QtGui/private/../../../../../../../../src/qt-dev_07.13-base_svg/qtbase/src/gui/painting/qfixed_p.h:90:58 in
   

Found by oss-fuzz as issue 36218. Google will publish the details in 89 days.