[REG 5.15.2 -> 6.0.0] undefined behavior in qmake

1. Configure Qt with "-sanitize undefined".
2. Build Qt.
3. Run qmake from this Qt build on a .pro file.
   On the command line, you'll see something like:

   /home/qtrob/dev/src/qt-dev-base_svg-11.19/qtbase/qmake/library/proitems.h:236:21: runtime error: null pointer passed as argument 2, which is declared to never be null
   /usr/include/string.h:44:28: note: nonnull attribute specified here
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/qtrob/dev/src/qt-dev-base_svg-11.19/qtbase/qmake/library/proitems.h:236:21 in 
   

With 5.15.2 this does not happen.