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>
(cherry picked from commit 5d3e9ababc0b3e049a7556b210bb6349e3e3322c)
Reviewed-on: https://gerrit.libreoffice.org/77696
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>
diff --git a/vcl/source/app/salvtables.cxx b/vcl/source/app/salvtables.cxx
index 88e1b1d..dd3a7f26 100644
--- a/vcl/source/app/salvtables.cxx
+++ b/vcl/source/app/salvtables.cxx
@@ -2073,7 +2073,7 @@ 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
        {