QSettings: Read past UTF-8 BOM even without textcodec support

Try to read past an UTF-8 BOM even if no textcodec support is available,
but do set the status to FormatError then.

This is in line with the general 'best effort' approach in QSettings.
It also allows qmake (that is built without textcodec support) to
gracefully load qt.conf files with a UTF-8 BOM, though non-Latin1
characters might still be misrepresented.

Fixes: QTBUG-83456
Change-Id: I31b45dc5a8adf7950910897a2b33ae16990d3818
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/corelib/io/qsettings.cpp

@@ -1673,14 +1673,16 @@ bool QConfFileSettingsPrivate::readIniFile(const QByteArray &data,
     int sectionPosition = 0;
     bool ok = true;
 
-#if QT_CONFIG(textcodec)
     // detect utf8 BOM
     const uchar *dd = (const uchar *)data.constData();
     if (data.size() >= 3 && dd[0] == 0xef && dd[1] == 0xbb && dd[2] == 0xbf) {
+#if QT_CONFIG(textcodec)
         iniCodec = QTextCodec::codecForName("UTF-8");
+#else
+        ok = false;
+#endif
         dataPos = 3;
     }
-#endif
 
     while (readIniLine(data, dataPos, lineStart, lineLen, equalsPos)) {
         char ch = data.at(lineStart);