ALSA: usb-audio: fix null pointer dereference on pointer cs_desc

mainline inclusion
from mainline-v5.16-rc1
commit b97053df
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9FNFQ
CVE: CVE-2021-47211

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b97053df0f04747c3c1e021ecbe99db675342954



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The pointer cs_desc return from snd_usb_find_clock_source could
be null, so there is a potential null pointer dereference issue.
Fix this by adding a null check before dereference.

Signed-off-by: Chengfeng Ye <cyeaa@connect.ust.hk>
Link: https://lore.kernel.org/r/20211024111736.11342-1-cyeaa@connect.ust.hk


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Conflicts:
	sound/usb/clock.c
Signed-off-by: Liu Shixin <liushixin2@huawei.com>