tdf#148197 gtk_tree_row_reference_new_proxy warning on docking navigator

Gtk-CRITICAL **: 09:05:11.124: gtk_tree_row_reference_new_proxy: assertion 'path->depth > 0' failed

from TreeView::set_cursor(-1)

gtk_tree_view_set_cursor is ok (and documented as such) with an
"invalid" path to unset the cursor, but there isn't the same for
gtk_tree_view_scroll_to_cell, though there null is docs as acceptable.

Change-Id: I11b94ba997fbbd2f31031d9e73765ea1882ad9ae
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/132872
Tested-by: Jenkins
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
diff --git a/vcl/unx/gtk3/gtkinst.cxx b/vcl/unx/gtk3/gtkinst.cxx
index 2732ea7..9c6327d 100644
--- a/vcl/unx/gtk3/gtkinst.cxx
+++ b/vcl/unx/gtk3/gtkinst.cxx
@@ -14552,7 +14552,7 @@ public:
    virtual void set_cursor(int pos) override
    {
        disable_notify_events();
        GtkTreePath* path = gtk_tree_path_new_from_indices(pos, -1);
        GtkTreePath* path = pos != -1 ? gtk_tree_path_new_from_indices(pos, -1) : nullptr;
        gtk_tree_view_scroll_to_cell(m_pTreeView, path, nullptr, false, 0, 0);
        gtk_tree_view_set_cursor(m_pTreeView, path, nullptr, false);
        gtk_tree_path_free(path);