qt: fix assertion in asyncFetchMore when cache tries to recursively refer items

The failling scenario is
* Count&Load task issued (task n°1)
* model need reset (thumbnail is updated for instance) => `needReload = true`
* Count&Load task (n°1) is resolved
  * diff util insert/remove rows
  * Callback from model change ask to refer an item beyond the cache
    * refer issue a new 'fetch more' request (n°2) as m_countTask is 0
  * as `needReload == true`, cache is moved to oldCache, new count&load request
    is issued (n° 3)
* task n°2 resolve, cache is null, assertion fails

by moving the `m_countTask` reset after the model update, we ensure that `refer`
won't start a new request while the model is still propagating the update.

modules/gui/qt/util/listcache.hxx

@@ -521,8 +521,6 @@ void ListCache<T>::asyncCountAndLoad()
             if (m_countTask != taskId)
                 return;
 
-            m_countTask = 0;
-
             //quite unlikley but model may change between count and load
             if (unlikely(list.size() > maximumCount))
             {
@@ -556,6 +554,8 @@ void ListCache<T>::asyncCountAndLoad()
                     emit localSizeChanged(0, m_cachedData->maximumCount);
             }
 
+            m_countTask = 0;
+
             if (m_needReload)
             {
                 m_needReload  = false;