QList::count(T): remove the &* hack on iterators

Use data() and data() + size() instead.

This solves an UB when the list is in its default-constructed state,
for which constData() == nullptr.

Pick-to: 6.2
Fixes: QTBUG-98770
Change-Id: I2cffe62afda945079b63fffd16bc165978c769f6
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/tools/qlist.h

@@ -585,7 +585,7 @@ public:
     template <typename AT = T>
     qsizetype count(const AT &t) const noexcept
     {
-        return qsizetype(std::count(&*cbegin(), &*cend(), t));
+        return qsizetype(std::count(data(), data() + size(), t));
     }
 
     void removeAt(qsizetype i) { remove(i); }