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

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.