tdf#121179 FILEOPEN: Crash opening a certain file (gtk/gtk3)

regression from
    commit 16b2b4f27acb83fc651b8484dead53ebd0e269e1
    Simplify containers iterations in sw/source/ui*

Change-Id: Id77c41acf1cb891ffa23ca1300633031e367d0d1
Reviewed-on: https://gerrit.libreoffice.org/62935
Reviewed-by: Arkadiy Illarionov <qarkai@gmail.com>
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/sw/source/uibase/docvw/frmsidebarwincontainer.cxx b/sw/source/uibase/docvw/frmsidebarwincontainer.cxx
index 5769386..f8abc3a 100644
--- a/sw/source/uibase/docvw/frmsidebarwincontainer.cxx
+++ b/sw/source/uibase/docvw/frmsidebarwincontainer.cxx
@@ -140,11 +140,13 @@
    if ( aFrameIter != mpFrameSidebarWinContainer->end() && nIndex >= 0 )
    {
        SidebarWinContainer& rSidebarWinContainer = (*aFrameIter).second;
        auto aIter = rSidebarWinContainer.begin();
        std::advance(aIter, nIndex);
        pRet = (*aIter).second;
        if (nIndex < sal_Int32(rSidebarWinContainer.size()))
        {
            auto aIter = rSidebarWinContainer.begin();
            std::advance(aIter, nIndex);
            pRet = (*aIter).second;
        }
    }

    return pRet;
}