cx231xx-cards: fix NULL-deref on missing association descriptor

cx231xx-cards: fix NULL-deref on missing association descriptor

commit 6c3b047fa2d2286d5e438bcb470c7b1a49f415f6 upstream.

Make sure to check that we actually have an Interface Association
Descriptor before dereferencing it during probe to avoid dereferencing a
NULL-pointer.

Fixes: e0d3bafd0258 ("V4L/DVB (10954): Add cx231xx USB driver")

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>