Fix bug in QDateTimeParser::findTimeZoneName()'s length-limiting

The recent commit 0c9fc20e7f missed the
case of a slash after some invalid character. That could lead it to
reinclude the invalid after previously working out it should ignore
it.

Change-Id: I3e29d2bf4d8df3878b581a969348ff5087b5d847
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Robert Löhning <robert.loehning@qt.io>

src/corelib/time/qdatetimeparser.cpp

@@ -1748,7 +1748,7 @@ QDateTimeParser::findTimeZoneName(QStringView str, const QDateTime &when) const
     Q_ASSERT(index <= str.size());
     while (lastSlash < index) {
         int slash = str.indexOf(QLatin1Char('/'), lastSlash + 1);
-        if (slash < 0)
+        if (slash < 0 || slash > index)
             slash = index; // i.e. the end of the candidate text
         else if (++count > 5)
             index = slash; // Truncate