QXmlStreamReader: make fastScanName() indicate parsing status to callers

QXmlStreamReader: make fastScanName() indicate parsing status to callers

Origin: upstream, commits
 https://code.qt.io/cgit/qt/qtbase.git/commit/?id=1a423ce4372d18a7
 https://code.qt.io/cgit/qt/qtbase.git/commit/?id=6326bec46a618c72
 https://code.qt.io/cgit/qt/qtbase.git/commit/?id=bdc8dc51380d2ce4
 https://code.qt.io/cgit/qt/qtbase.git/commit/?id=3bc3b8d69a291aa5
 .
 Based on KDE's backport:
 https://invent.kde.org/qt/qt/qtbase/-/merge_requests/263
Last-Update: 2023-07-15

This fixes a crash while parsing an XML file with garbage data, the file
starts with '<' then garbage data:
- The loop in the parse() keeps iterating until it hits "case 262:",
  which calls fastScanName()
- fastScanName() iterates over the text buffer scanning for the
  attribute name (e.g. "xml:lang"), until it finds ':'
- Consider a Value val, fastScanName() is called on it, it would set
  val.prefix to a number > val.len, then it would hit the 4096 condition
  and return (returned 0, now it returns the equivalent of
  std::null_opt), which means that val.len doesn't get modified, making
  it smaller than val.prefix
- The code would try constructing an XmlStringRef with negative length,
  which would hit an assert in one of QStringView's constructors

Add an assert to the XmlStringRef constructor.

Add unittest based on the file from the bug report.

Credit to OSS-Fuzz.

Gbp-Pq: Name CVE-2023-37369.diff