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 long snd_seq_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
	struct snd_seq_client *client = file->private_data;

	if (snd_BUG_ON(!client))
		return -ENXIO;
		
	return snd_seq_do_ioctl(client, cmd, (void __user *) arg);
}

static long snd_seq_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
	struct snd_seq_client *client = file->private_data;

	if (snd_BUG_ON(!client))
		return -ENXIO;
		
	return snd_seq_do_ioctl(client, cmd, (void __user *) arg);
}
