[subset] avoid calling clear on null pool set.

src/hb-ot-layout-gsubgpos.hh

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