ALSA: usb-audio: fix memory leak on cval

With the current exit return path of the ctl_info allocation failure
cval is not being freed resulting in a memory leak. Fix this by kfree'ing
it on the return.

Detected by CoverityScan, CID#1466878 ("Resource Leak")

Fixes: 21e9b3e9 ("ALSA: usb-audio: fix uac control query argument")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -1365,8 +1365,10 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
 	cval->cmask = ctl_mask;
 
 	ctl_info = get_feature_control_info(control);
-	if (!ctl_info)
+	if (!ctl_info) {
+		kfree(cval);
 		return;
+	}
 	if (state->mixer->protocol == UAC_VERSION_1)
 		cval->val_type = ctl_info->type;
 	else /* UAC_VERSION_2 */