[subset] Fix fuzzer found memory leak.

Happens because an insert into a map with an invalid key reports successful, but this causes the set being inserted to be lost.

src/hb-ot-layout-gsubgpos.hh

@@ -3694,6 +3694,7 @@ struct GSUBGPOS
     for (unsigned i : feature_indices->iter ())
     {
       hb_tag_t t = get_feature_tag (i);
+      if (t == unique_features.INVALID_KEY) continue;
       if (!unique_features.has (t))
       {
         hb_set_t* indices = hb_set_create ();