QImage::fill stops working on "big" images because of int overflow

The attach code fails to fill all the image as white because 46341 * 46341 is bigger than MAX_INT

The overflow happens in qtbase/src/gui/painting/qdrawhelper_p.h:940:58

   qt_memfill(reinterpret_cast<T*>(d), value, width * height);

Found while fuzzing kimageformats.

You may want to mention oss-fuzz/12762 that is the bug number assigned to it. https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12762 (Will be public in 90 days)