ALSA: usb-audio: Fix NULL dereference at parsing BADD

snd_usb_mixer_controls_badd() that parses UAC3 BADD profiles misses a
NULL check for the given interfaces.  When a malformed USB descriptor
is passed, this may lead to an Oops, as spotted by syzkaller.
Skip the iteration if the interface doesn't exist for avoiding the
crash.

Fixes: 17156f23 ("ALSA: usb: add UAC3 BADD profiles support")
Reported-by:  <syzbot+a36ab65c6653d7ccdd62@syzkaller.appspotmail.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191122112840.24797-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>