Integer-overflow in QTextHtmlParser::margin

1. Have a build of Qt configured with "-sanitize undefined".
2. Build the attached project.
3. Run the resulting program and pass the input file:

   ./report input.html
   

   You will see output like:

   /home/qtrob/dev/src/qt-5.15.2-base_svg/qtbase/src/gui/text/qtexthtmlparser.cpp:608:15: runtime error: signed integer overflow: 2147483647 + 2147483647 cannot be represented in type 'int'
   SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/qtrob/dev/src/qt-5.15.2-base_svg/qtbase/src/gui/text/qtexthtmlparser.cpp:608:15 in
   

Found by oss-fuzz as issue 25867.