aout: don't update the clock while stopping
This fixes the following assert:
src/player/timer.c:301: void vlc_player_UpdateTimer(vlc_player_t *, vlc_es_id_t *, _Bool, const struct vlc_player_timer_point *, vlc_tick_t, unsigned int, unsigned int): assertion "point->ts != VLC_TICK_INVALID" failed