fix resolver crash when DNS server unreachable

src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.cc

@@ -153,7 +153,10 @@ static void grpc_ares_request_unref(grpc_ares_request* r) {
   /* If there are no pending queries, invoke on_done callback and destroy the
      request */
   if (gpr_unref(&r->pending_queries)) {
-    grpc_cares_wrapper_address_sorting_sort(*(r->lb_addrs_out));
+    grpc_lb_addresses* lb_addrs = *(r->lb_addrs_out);
+    if (lb_addrs != nullptr) {
+      grpc_cares_wrapper_address_sorting_sort(lb_addrs);
+    }
     GRPC_CLOSURE_SCHED(r->on_done, r->error);
     gpr_mu_destroy(&r->mu);
     grpc_ares_ev_driver_destroy(r->ev_driver);