Calling QWebEngineHistory::currentItem() before the first item is
inserted, caused a segfault, due to accessing an illegal QList index.
We now delegate the lookup to QWebEngineHistory::itemAt(), which checks
the index first and returns a dummy element in case of an illegal
index.

Task-number: QTBUG-59599
Change-Id: I9cdd2533e33415a7b812c29fff5429eb005020dc
Reviewed-by: Michael Brüning <michael.bruning@qt.io>