[metrics] Fall back to hhea if OS2 metrics are empty

Reinstates previous logic, even if it might be unnecessary.

src/hb-ot-metrics-internal.cc

@@ -60,14 +60,14 @@ hb_ot_metrics_get_position_internal (hb_face_t       *face,
   (face->table.TABLE->has_data () && \
     (position && (*position = face->table.TABLE->ATTR + GET_VAR), true))
   case HB_OT_METRICS_HORIZONTAL_ASCENDER:
-    return face->table.OS2->use_typo_metrics () ?
-	   GET_METRIC (OS2, sTypoAscender) : GET_METRIC (hhea, ascender);
+    return (face->table.OS2->use_typo_metrics () && GET_METRIC (OS2, sTypoAscender)) ||
+	   GET_METRIC (hhea, ascender);
   case HB_OT_METRICS_HORIZONTAL_DESCENDER:
-    return face->table.OS2->use_typo_metrics () ?
-	   GET_METRIC (OS2, sTypoDescender) : GET_METRIC (hhea, descender);
+    return (face->table.OS2->use_typo_metrics () && GET_METRIC (OS2, sTypoDescender)) ||
+	   GET_METRIC (hhea, descender);
   case HB_OT_METRICS_HORIZONTAL_LINE_GAP:
-    return face->table.OS2->use_typo_metrics () ?
-	   GET_METRIC (OS2, sTypoLineGap) : GET_METRIC (hhea, lineGap);
+    return (face->table.OS2->use_typo_metrics () && GET_METRIC (OS2, sTypoLineGap)) ||
+	   GET_METRIC (hhea, lineGap);
   case HB_OT_METRICS_HORIZONTAL_CLIPPING_ASCENT:  return GET_METRIC (OS2, usWinAscent);
   case HB_OT_METRICS_HORIZONTAL_CLIPPING_DESCENT: return GET_METRIC (OS2, usWinDescent);
   case HB_OT_METRICS_VERTICAL_ASCENDER:           return GET_METRIC (vhea, ascender);