Resolves: tdf#126951 crash in custom slide show

on selecting entry 0 which doesn't exist

Change-Id: I67656bed63f9055bc6c5dd5781e79f967996500d
Reviewed-on: https://gerrit.libreoffice.org/77560
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/vcl/source/app/salvtables.cxx b/vcl/source/app/salvtables.cxx
index 3dfe6b2..0ca121c 100644
--- a/vcl/source/app/salvtables.cxx
+++ b/vcl/source/app/salvtables.cxx
@@ -3094,7 +3094,7 @@ public:
    {
        assert(m_xTreeView->IsUpdateMode() && "don't select when frozen");
        disable_notify_events();
        if (pos == -1)
        if (pos == -1 || (pos == 0 && n_children() == 0))
            m_xTreeView->SelectAll(false);
        else
        {