ALSA: timer - Add NULL-check for invalid slave timer

Just to be sure.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer.c

@@ -328,6 +328,8 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 		mutex_unlock(&register_mutex);
 	} else {
 		timer = timeri->timer;
+		if (snd_BUG_ON(!timer))
+			goto out;
 		/* wait, until the active callback is finished */
 		spin_lock_irq(&timer->lock);
 		while (timeri->flags & SNDRV_TIMER_IFLG_CALLBACK) {
@@ -353,6 +355,7 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 		}
 		mutex_unlock(&register_mutex);
 	}
+ out:
 	if (timeri->private_free)
 		timeri->private_free(timeri);
 	kfree(timeri->owner);