Fixed crash when clearing history.

To reproduce the issue:
* clear your browsing history via the ui - do not close the browser
* start navigating pages, so that new items get inserted into the
* history model.
-> will crash as m_todayItem got deleted but not recreated, as it was
not NULLed.

src/lib/history/historymodel.cpp

@@ -264,6 +264,7 @@ void HistoryModel::resetHistory()
     beginResetModel();
 
     delete m_rootItem;
+    m_todayItem = 0;
     m_rootItem = new HistoryItem(0);
 
     init();