ALSA: usb-audio: fix sync-ep altsetting sanity check

The altsetting sanity check in set_sync_ep_implicit_fb_quirk() was
checking for there to be at least one altsetting but then went on to
access the second one, which may not exist.

This could lead to random slab data being used to initialise the sync
endpoint in snd_usb_add_endpoint().

Fixes: c75a8a7a ("ALSA: snd-usb: add support for implicit feedback")
Fixes: ca10a7eb ("ALSA: usb-audio: FT C400 sync playback EP to capture EP")
Fixes: 5e35dc03 ("ALSA: usb-audio: add implicit fb quirk for Behringer UFX1204")
Fixes: 17f08b0d ("ALSA: usb-audio: add implicit fb quirk for Axe-Fx II")
Fixes: 103e9625 ("ALSA: usb-audio: simplify set_sync_ep_implicit_fb_quirk")
Cc: stable <stable@vger.kernel.org>     # 3.5
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20200114083953.1106-1-johan@kernel.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -370,7 +370,7 @@ static int set_sync_ep_implicit_fb_quirk(struct snd_usb_substream *subs,
 add_sync_ep_from_ifnum:
 	iface = usb_ifnum_to_if(dev, ifnum);
 
-	if (!iface || iface->num_altsetting == 0)
+	if (!iface || iface->num_altsetting < 2)
 		return -EINVAL;
 
 	alts = &iface->altsetting[1];