fix dns crash

src/core/dns.c

@@ -1064,7 +1064,11 @@ dns_check_entry(u8_t i)
       entry->server_idx = 0;
       entry->tmr = 1;
       entry->retries = 0;
-
+#if ESP_DNS
+      while((entry->server_idx < DNS_MAX_SERVERS) && ip_addr_isany_val(dns_servers[entry->server_idx])) {
+        entry->server_idx++;
+      }
+#endif
       /* send DNS packet for this entry */
       err = dns_send(i);
       if (err != ERR_OK) {