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>
int snd_seq_kernel_client_ctl(int clientid, unsigned int cmd, void *arg)
{
	struct snd_seq_client *client;
	mm_segment_t fs;
	int result;

	client = clientptr(clientid);
	if (client == NULL)
		return -ENXIO;
	fs = snd_enter_user();
	result = snd_seq_do_ioctl(client, cmd, (void __force __user *)arg);
	snd_leave_user(fs);
	return result;
}

int snd_seq_kernel_client_ctl(int clientid, unsigned int cmd, void *arg)
{
	struct snd_seq_client *client;
	mm_segment_t fs;
	int result;

	client = clientptr(clientid);
	if (client == NULL)
		return -ENXIO;
	fs = snd_enter_user();
	result = snd_seq_do_ioctl(client, cmd, (void __force __user *)arg);
	snd_leave_user(fs);
	return result;
}
