[vte/wip/egmont/bidi] fix a crash typically on alternate screen top row

[Egmont Koblinger <egmontkob src gnome org>]

commit 505cdf9829c76e39228cad4631b4cd2cd55ee3a5
Author: Egmont Koblinger <egmont gmail com>
Date:   Fri Sep 14 00:27:14 2018 +0200

    fix a crash typically on alternate screen top row

 src/vte.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/vte.cc b/src/vte.cc
index 9332ed6f..fe1aa4e4 100644
--- a/src/vte.cc
+++ b/src/vte.cc
@@ -3036,7 +3036,7 @@ Terminal::maybe_apply_bidi_attributes()
 
         auto row = m_screen->cursor.row;
 
-        if (row > 0) {
+        if (row > _vte_ring_delta (m_screen->row_data)) {
                 const VteRowData *rowdata = _vte_ring_index (m_screen->row_data, row - 1);
                 if (rowdata != nullptr && rowdata->attr.soft_wrapped) {
                         _vte_debug_print(VTE_DEBUG_BIDI,