[REG 5.15.2 -> 6.0.0] QTextDocument::setMarkdown very slow on some input

1. Build the attached project.
   No sanitizers needed.
2. Run the resulting program passing in one of the attached input files, e.g.:

   ./report 29381.md
   

It takes nearly two minutes until the program returns. If the file is broken, could QTextDocument notice that and exit earlier?

Google's oss-fuzz found this as issue 29381.