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 multicast_event(struct snd_seq_client *client, struct snd_seq_event *event,
			   int atomic, int hop)
{
	pr_debug("ALSA: seq: multicast not supported yet.\n");
	return 0; /* ignored */
}

static int multicast_event(struct snd_seq_client *client, struct snd_seq_event *event,
			   int atomic, int hop)
{
	pr_debug("ALSA: seq: multicast not supported yet.\n");
	return 0; /* ignored */
}
