[subset] don't visit lookup if covered glyph set has failed.

If covered glyph set is in error then the same lookup can be recursed into repeatedly potentially causing a fuzzer timeout. Fixes: https://oss-fuzz.com/testcase-detail/5416421032067072.

src/hb-ot-layout-gsubgpos.hh

@@ -120,6 +120,8 @@ struct hb_closure_context_t :
     }
 
     hb_set_t *covered_glyph_set = done_lookups_glyph_set->get (lookup_index);
+    if (covered_glyph_set->in_error ())
+      return true;
     if (parent_active_glyphs ()->is_subset (covered_glyph_set))
       return true;