CVE-2025-3512: Qt Base QTextMarkdownImporter Front Matter Buffer Overflow

["田世林" <tianshilin () deepin org>]

A heap buffer overflow vulnerability exists in `QTextMarkdownImporter`.
When parsing the front matter of a Markdown file, the code assumes that
more characters (e.g., a newline) will be present in the input after
finding the closing marker `---`. However, if the input stream ends with
the `----` delimiter and lacks a trailing newline, calling
`QStringView::sliced()` will attempt to access characters beyond the end
of the string, causing the program to crash.

Causes of the vulnerability:

1. Insufficient boundary checks: The case where the delimiter appears at
the end of the input was not handled correctly.
2. Overly permissive marker requirements: Allowing unexpected whitespace
or other formatting errors led to issues in the parsing logic.

   Affected versions: 6.8.0 to 6.8.3
   Unaffected versions: Versions prior to 6.6.0

   Fix patch: https://codereview.qt-project.org/c/qt/qtbase/+/635546