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

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

BugLink: https://bugs.launchpad.net/bugs/1952579
[ Upstream commit b97053df0f04747c3c1e021ecbe99db675342954 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>