gtk_tree_model_iter_parent modifies the 1st arg, not 2nd

Change-Id: I015022c4dd68f152ef5e56947031ccf4bfaf2b1c
Reviewed-on: https://gerrit.libreoffice.org/71516
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -7000,7 +7000,6 @@ public:
         }
         if (!gtk_tree_model_iter_parent(pModel, &tmp, &iter))
             return false;
-        tmp = iter;
         if (gtk_tree_model_iter_next(pModel, &tmp))
         {
             rGtkIter.iter = tmp;