[REG 6.3.2 -> 6.4.0] toString() on valid QDateTime causes undefined behavior

1. Have a build of Qt configured with "-sanitize undefined".
2. Use that to build the attached project.
   The output will include something like:

   /home/qtrob/dev/src/qt-dev_01.02-base_imageformats_svg/qtbase/src/corelib/time/qdatetime.cpp:3061:60: runtime error: signed integer overflow: -106751991168 * 86400000 cannot be represented in type 'long long'
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/qtrob/dev/src/qt-dev_01.02-base_imageformats_svg/qtbase/src/corelib/time/qdatetime.cpp:3061:60 in 
   /home/qtrob/dev/src/qt-dev_01.02-base_imageformats_svg/qtbase/src/corelib/time/qdatetime.cpp:3061:53: runtime error: signed integer overflow: -9223372036838297000 - 9223372036794351616 cannot be represented in type 'long long'
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/qtrob/dev/src/qt-dev_01.02-base_imageformats_svg/qtbase/src/corelib/time/qdatetime.cpp:3061:53 in
   

I expect that a valid QDateTime can be converted to a QString without undefined behavior.

Google's oss-fuzz found this as issue 47607.