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

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

sound/usb/clock.c

@@ -496,6 +496,10 @@ int snd_usb_set_sample_rate_v2v3(struct snd_usb_audio *chip,
 	union uac23_clock_source_desc *cs_desc;
 
 	cs_desc = snd_usb_find_clock_source(chip, clock, fmt->protocol);
+
+	if (!cs_desc)
+		return 0;
+
 	if (fmt->protocol == UAC_VERSION_3)
 		bmControls = le32_to_cpu(cs_desc->v3.bmControls);
 	else