zvbi: return actual last visible row

Currently in most cases the row after the last is returned.
When the last row is visible vbi_draw_vt_page_region() could
crash, because it went over the size of p_page.text and use
invalid values (>=40) as index for p_page.color_map.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/zvbi.c

@@ -605,7 +605,7 @@ static int get_last_visible_row( vbi_char *p_text, int rows, int columns)
     {
         if (p_text[i].opacity != VBI_TRANSPARENT_SPACE)
         {
-            return ( i + columns - 1) / columns;
+            return i / columns;
         }
     }