[subset] Fix fuzzer issue.

Fixes https://oss-fuzz.com/testcase-detail/5693568490012672. new_index should be set from new_index2 when the entry is present in the map.

src/hb-ot-post-table-v2subset.hh

@@ -52,11 +52,11 @@ HB_INTERNAL bool postV2Tail::serialize (hb_serialize_context_t *c,
   {
     unsigned glyph_id = _.first;
     unsigned new_index = _.second;
-    
+
     if (new_index < 258) continue;
     if (copied_indices.has (new_index)) continue;
     copied_indices.add (new_index);
-    
+
     hb_bytes_t s = reinterpret_cast<const post::accelerator_t*> (_post)->find_glyph_name (glyph_id);
     HBUINT8 *o = c->allocate_size<HBUINT8> (HBUINT8::static_size * (s.length + 1));
     if (unlikely (!o)) return_trace (false);
@@ -87,9 +87,10 @@ HB_INTERNAL bool postV2Tail::subset (hb_subset_context_t *c) const
     unsigned new_index;
     const unsigned *new_index2;
     if (old_index <= 257) new_index = old_index;
-    else if (!old_new_index_map.has (old_index, &new_index2))
+    else if (old_new_index_map.has (old_index, &new_index2))
     {
       new_index = *new_index2;
+    } else {
       hb_bytes_t s = _post.find_glyph_name (old_gid);
       new_index = glyph_name_to_new_index.get (s);
       if (new_index == (unsigned)-1)