BUG/MINOR: mqtt: connack parser returns MQTT_NEED_MORE_DATA on unknown property

In mqtt_parse_connack(), the switch statement's default case for unknown
MQTT properties was using 'return 0' which returns MQTT_NEED_MORE_DATA.
This is misleading: an unknown property should be treated as an invalid
message (MQTT_INVALID_MESSAGE), like other functions do. This branches to
the "end" label without touching the preset return value instead. This can
be backported if needed.

src/mqtt.c

@@ -1196,7 +1196,7 @@ static int mqtt_parse_connack(struct ist parser, struct mqtt_pkt *mpkt)
 				break;
 
 			default:
-				return 0;
+				goto end;
 			}
 
 			if (!isttest(props))