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_novation_boot_quirk(struct usb_device *dev)
{
	/* preemptively set up the device because otherwise the
	 * raw MIDI endpoints are not active */
	usb_set_interface(dev, 0, 1);
	return 0;
}

static int snd_usb_novation_boot_quirk(struct usb_device *dev)
{
	/* preemptively set up the device because otherwise the
	 * raw MIDI endpoints are not active */
	usb_set_interface(dev, 0, 1);
	return 0;
}
