fix: It sometimes crashes after disable sntp

Author: xcguang

third_party/lwip/core/sntp.c

@@ -919,6 +919,10 @@ sntp_dns_found(const char* hostname, ip_addr_t *ipaddr, void *arg)
  LWIP_UNUSED_ARG(hostname);
  LWIP_UNUSED_ARG(arg);
 
+ if (sntp_pcb == NULL) {
+ return;
+ }
+ 
  if (ipaddr != NULL) {
  /* Address resolved, send request */
  LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_dns_found: Server address resolved, sending request\n"));