ensure valid iterator even with empty content.

Change-Id: Icd2e193b66ace8212d1af086192abf0c43c5953f
diff --git a/sfx2/source/sidebar/SidebarController.cxx b/sfx2/source/sidebar/SidebarController.cxx
index 9f78d58..ab01e7d 100644
--- a/sfx2/source/sidebar/SidebarController.cxx
+++ b/sfx2/source/sidebar/SidebarController.cxx
@@ -621,6 +621,7 @@ void SidebarController::SwitchToDeck (
        }
        else
        {
            iPanel = rCurrentPanels.end();
            for (auto a = rCurrentPanels.begin(); a != rCurrentPanels.end(); ++a)
            {
                iPanel = a;