Don't crash under certain weird conditions.

In case of 'version 1' navigation settings, and only a 'Open Documents'
panel visible, we ended up in an endless loop.

src/plugins/coreplugin/navigationwidget.cpp

@@ -272,7 +272,7 @@ void NavigationWidget::restoreSettings(QSettings *settings)
     } else {
         QList<int> sizes;
         sizes += 256;
-        for (int i = views.size()-1; i; --i)
+        for (int i = views.size()-1; i > 0; --i)
             sizes.prepend(512);
         setSizes(sizes);
     }