ALSA: seq: Fix missing NULL check at remove_events ioctl

snd_seq_ioctl_remove_events() calls snd_seq_fifo_clear()
unconditionally even if there is no FIFO assigned, and this leads to
an Oops due to NULL dereference.  The fix is just to add a proper NULL
check.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
static int snd_seq_ioctl_get_client_info(struct snd_seq_client *client,
					 void __user *arg)
{
	struct snd_seq_client *cptr;
	struct snd_seq_client_info client_info;

	if (copy_from_user(&client_info, arg, sizeof(client_info)))
		return -EFAULT;

	/* requested client number */
	cptr = snd_seq_client_use_ptr(client_info.client);
	if (cptr == NULL)
		return -ENOENT;		/* don't change !!! */

	get_client_info(cptr, &client_info);
	snd_seq_client_unlock(cptr);

	if (copy_to_user(arg, &client_info, sizeof(client_info)))
		return -EFAULT;
	return 0;
}

static int snd_seq_ioctl_get_client_info(struct snd_seq_client *client,
					 void __user *arg)
{
	struct snd_seq_client *cptr;
	struct snd_seq_client_info client_info;

	if (copy_from_user(&client_info, arg, sizeof(client_info)))
		return -EFAULT;

	/* requested client number */
	cptr = snd_seq_client_use_ptr(client_info.client);
	if (cptr == NULL)
		return -ENOENT;		/* don't change !!! */

	get_client_info(cptr, &client_info);
	snd_seq_client_unlock(cptr);

	if (copy_to_user(arg, &client_info, sizeof(client_info)))
		return -EFAULT;
	return 0;
}
