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;