Fix possible crash having just upgraded from 1.6.

Happens if `per_listener_settings true` is set, and a SIGHUP is sent to
the broker before a client has reconnected to the broker.

Closes #2167. Thanks to momoskitto.

ChangeLog.txt

@@ -1,6 +1,11 @@
 2.0.11 - 2021-xx-xx
 ===================
 
+Broker:
+- Fix possible crash having just upgraded from 1.6 if `per_listener_settings
+ true` is set, and a SIGHUP is sent to the broker before a client has
+ reconnected to the broker. Closes #2167.
+
 Clients:
 - If sending mosquitto_sub output to a pipe, mosquitto_sub will now detect
  that the pipe has closed and disconnect. Closes #2164.

src/security.c

@@ -764,6 +764,9 @@ int mosquitto_unpwd_check(struct mosquitto *context)
  rc = MOSQ_ERR_PLUGIN_DEFER;
 
  if(db.config->per_listener_settings){
+ if(context->listener == NULL){
+ return MOSQ_ERR_AUTH;
+ }
  opts = &context->listener->security_options;
  }else{
  opts = &db.config->security_options;