ALSA: usb-audio: Fix NULL dereference in create_fixed_stream_quirk()

create_fixed_stream_quirk() may cause a NULL-pointer dereference by
accessing the non-existing endpoint when a USB device with a malformed
USB descriptor is used.

This patch avoids it simply by adding a sanity check of bNumEndpoints
before the accesses.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=971125
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
static int snd_usb_accessmusic_boot_quirk(struct usb_device *dev)
{
	int err, actual_length;

	/* "midi send" enable */
	static const u8 seq[] = { 0x4e, 0x73, 0x52, 0x01 };

	void *buf = kmemdup(seq, ARRAY_SIZE(seq), GFP_KERNEL);
	if (!buf)
		return -ENOMEM;
	err = usb_interrupt_msg(dev, usb_sndintpipe(dev, 0x05), buf,
			ARRAY_SIZE(seq), &actual_length, 1000);
	kfree(buf);
	if (err < 0)
		return err;

	return 0;
}

static int snd_usb_accessmusic_boot_quirk(struct usb_device *dev)
{
	int err, actual_length;

	/* "midi send" enable */
	static const u8 seq[] = { 0x4e, 0x73, 0x52, 0x01 };

	void *buf = kmemdup(seq, ARRAY_SIZE(seq), GFP_KERNEL);
	if (!buf)
		return -ENOMEM;
	err = usb_interrupt_msg(dev, usb_sndintpipe(dev, 0x05), buf,
			ARRAY_SIZE(seq), &actual_length, 1000);
	kfree(buf);
	if (err < 0)
		return err;

	return 0;
}
