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

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

commit 0f886ca12765d20124bd06291c82951fd49a33be upstream.

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
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.2:
 - There's no altsd variable
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>