Integer-overflow in QTextHtmlImporter::scanTable

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 26123.html
   

   You will see output like:

   /src/qt/qtbase/src/gui/text/qtextdocumentfragment.cpp:996:23: runtime error: signed integer overflow: 2147483647 + 40 cannot be represented in type 'int'
   

Found by oss-fuzz as issue 26123.