[REG 5.15.1 -> 6.0] Crash in QTextDocument().setMarkdown()

1. Have a build of Qt configured with "-release -sanitize address".
2. Use that to build the attached project.
3. Pass the attached markdown file to the resulting program:

   ./report newinput.md
   

   It will crash with output like:

   AddressSanitizer:DEADLYSIGNAL
   =================================================================
   ==49288==ERROR: AddressSanitizer: SEGV on unknown address 0x000402445180 (pc 0x0000013dd71e bp 0x7ffeae983010 sp 0x7ffeae982ba0 T0)
   ==49288==The signal is caused by a READ memory access.
       #0 0x13dd71e in md_process_doc (/tmp/build-report/report+0x13dd71e)
       #1 0x13d842e in md_parse (/tmp/build-report/report+0x13d842e)
       #2 0xc99dfe in QTextMarkdownImporter::import(QTextDocument*, QString const&) (/tmp/build-report/report+0xc99dfe)
       #3 0xb34600 in QTextDocument::setMarkdown(QString const&, QFlags<QTextDocument::MarkdownFeature>) (/tmp/build-report/report+0xb34600)
       #4 0x4e151a in main (/tmp/build-report/report+0x4e151a)
       #5 0x7fd99ef1f0b2 in __libc_start_main /build/glibc-ZN95T4/glibc-2.31/csu/../csu/libc-start.c:308:16
       #6 0x4368dd in _start (/tmp/build-report/report+0x4368dd)
   

Qt build from 5.15 branch does not seem to crash in this situation.