fix test-ot-face crash

src/hb-ot-cff1-table.hh

@@ -1267,6 +1267,7 @@ struct cff1
 			      char *buf, unsigned int buf_len) const
     {
       if (!buf) return true;
+      if (unlikely (!is_valid ())) return false;
       hb_codepoint_t sid = glyph_to_sid (glyph);
       byte_str_t byte_str;
       const char *str;