ALSA: usb-audio: Add sanity checks for endpoint accesses

ALSA: usb-audio: Add sanity checks for endpoint accesses

commit 447d6275f0c21f6cc97a88b3a0c601436a4cdf2a upstream.

Add some sanity check codes before actually accessing the endpoint via
get_endpoint() in order to avoid the invalid access through a
malformed USB descriptor.  Mostly just checking bNumEndpoints, but in
one place (snd_microii_spdif_default_get()), the validity of iface and
altsetting index is checked as well.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=971125
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2: drop changes to code we don't have]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>