silence warnings about negative widths under RTL

I don't see any visual problems, but there are these
annoying warnings

Change-Id: Ifc8830503cdc57084272a973b09927069caabada
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106713
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
diff --git a/vcl/unx/gtk3/gtk3gtkframe.cxx b/vcl/unx/gtk3/gtk3gtkframe.cxx
index 4680acf..bb636e7 100644
--- a/vcl/unx/gtk3/gtk3gtkframe.cxx
+++ b/vcl/unx/gtk3/gtk3gtkframe.cxx
@@ -2953,6 +2953,10 @@ void GtkSalFrame::damaged(sal_Int32 nExtentsX, sal_Int32 nExtentsY,
    }
#endif

    // quite a bit of noise in RTL mode with negative widths
    if (nExtentsWidth <= 0 || nExtentsHeight <= 0)
        return;

    gtk_widget_queue_draw_area(GTK_WIDGET(m_pFixedContainer),
                               nExtentsX, nExtentsY,
                               nExtentsWidth, nExtentsHeight);