Fix potential crash when reloading config.

Closes #1424, #1425. Thanks to JinPingChng and peteakalad.

ChangeLog.txt

@@ -3,6 +3,7 @@
 
 Broker:
 - Add workaround for working with libwebsockets 3.2.0.
+- Fix potential crash when reloading config. Closes #1424, #1425.
 
 Client library:
 - Don't use `/` in autogenerated client ids, to avoid confusing with topics.

src/security_default.c

@@ -1012,7 +1012,7 @@ int mosquitto_security_apply_default(struct mosquitto_db *db)
 
 		/* Check for connected clients that are no longer authorised */
 #ifdef WITH_TLS
-		if(context->listener->ssl_ctx && (context->listener->use_identity_as_username || context->listener->use_subject_as_username)){
+		if(context->listener && context->listener->ssl_ctx && (context->listener->use_identity_as_username || context->listener->use_subject_as_username)){
 			/* Client must have either a valid certificate, or valid PSK used as a username. */
 			if(!context->ssl){
 				if(context->protocol == mosq_p_mqtt5){