[subset] use hb_set_clear to avoid calling clear() on null pool set.

src/hb-ot-layout-gsubgpos.hh

@@ -116,8 +116,7 @@ struct hb_closure_context_t :
 	}
       }
 
-      if (done_lookups_glyph_set->get (lookup_index)->get_population ())
-        done_lookups_glyph_set->get (lookup_index)->clear ();
+      hb_set_clear (done_lookups_glyph_set->get (lookup_index));
     }
 
     hb_set_t *covered_glyph_set = done_lookups_glyph_set->get (lookup_index);