If a markdown file has FrontMatter, we look for the end of the newlines
after the `---` marker to begin parsing the actual markdown. But check
bounds in case the file contains only front matter and not markdown.
Fixes: QTBUG-122982
Change-Id: I09c4ae90c47ebd84877738aecc1d1cad0b0bfca2
Reviewed-by: Anton Kudryavtsev <antkudr@mail.ru>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>