Merge branch 'bugfix/N8266-67' into 'master'

fix: It sometimes crashes after disable sntp

See merge request sdk/ESP8266_NONOS_SDK!281

Author: xcguang

VERSION

@@ -17,6 +17,6 @@ phy:
 gitlab:
     driver      :   5a2a54b4
     json        :   f55d5fee
-    lwip        :   5c01bdee
+    lwip        :   5623f48f
     mbedtls     :   82b93fe5
     smartconfig :   ea1d4a13(v2.5.5)

lib/liblwip.a

@@ -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"));