[REG 6.5.3 -> 6.6.0] Null-pointer deref when loading svg file

1. Have a build of Qt including qtsvg.
   No sanitizers needed.
2. Build the attached project:

   qt-cmake /tmp/report/ && cmake --build .
   

3. Run the resulting binary passing the attached input file as parameter:

   ./report /tmp/report/61586.svg
   

   The program crashes when dereferencing a null pointer:

   1  QHash<QString, QSvgRefCounter<QSvgFont>>::valueImpl<QString, QHash<QString, QSvgRefCounter<QSvgFont>>::value(QString const&) const::{lambda()#1}>(QString const&, QHash<QString, QSvgRefCounter<QSvgFont>>::value(QString const&) const::{lambda()#1}&&) const qhash.h              1046 0x7ffff7f7afcf 
   2  QHash<QString, QSvgRefCounter<QSvgFont>>::value                                                                                                                                                                                                                qhash.h              1056 0x7ffff7f7af6d 
   3  QHash<QString, QSvgRefCounter<QSvgFont>>::operator[]                                                                                                                                                                                                           qhash.h              1083 0x7ffff7f77daf 
   4  QSvgTinyDocument::svgFont                                                                                                                                                                                                                                      qsvgtinydocument.cpp 363  0x7ffff7f769ad 
   5  parseFont                                                                                                                                                                                                                                                      qsvghandler.cpp      1353 0x7ffff7f114f3 
   6  parseStyle                                                                                                                                                                                                                                                     qsvghandler.cpp      2386 0x7ffff7f0fc08 
   7  parseStyle                                                                                                                                                                                                                                                     qsvghandler.cpp      2429 0x7ffff7f05896 
   8  cssStyleLookup                                                                                                                                                                                                                                                 qsvghandler.cpp      2071 0x7ffff7f0f60d 
   9  parseStopNode                                                                                                                                                                                                                                                  qsvghandler.cpp      3879 0x7ffff7f22823 
   10 QSvgHandler::startElement                                                                                                                                                                                                                                      qsvghandler.cpp      4776 0x7ffff7f02b98 
   11 QSvgHandler::parse                                                                                                                                                                                                                                             qsvghandler.cpp      4571 0x7ffff7f00ef3 
   12 QSvgHandler::init                                                                                                                                                                                                                                              qsvghandler.cpp      4471 0x7ffff7f00b24 
   13 QSvgHandler::QSvgHandler                                                                                                                                                                                                                                       qsvghandler.cpp      4444 0x7ffff7f00a24 
   14 QSvgTinyDocument::load                                                                                                                                                                                                                                         qsvgtinydocument.cpp 214  0x7ffff7f75427 
   15 loadDocument<QByteArray>                                                                                                                                                                                                                                       qsvgrenderer.cpp     400  0x7ffff7f5391a 
   16 QSvgRenderer::load                                                                                                                                                                                                                                             qsvgrenderer.cpp     430  0x7ffff7f52f04 
   17 QSvgRenderer::QSvgRenderer                                                                                                                                                                                                                                     qsvgrenderer.cpp     146  0x7ffff7f52eb6 
   18 main                                                                                                                                                                                                                                                           main.cpp             13   0x555555555354 
   

Google's oss-fuzz found this as issue 61586.