sound: oxygen: fix NULL pointer dereference when loading snd-oxygen

Check that model->control_filter is set before trying to call it.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/oxygen/oxygen_mixer.c

@@ -936,11 +936,13 @@ static int add_controls(struct oxygen *chip,
 
 	for (i = 0; i < count; ++i) {
 		template = controls[i];
+		if (chip->model->control_filter) {
 			err = chip->model->control_filter(&template);
 			if (err < 0)
 				return err;
 			if (err == 1)
 				continue;
+		}
 		if (!strcmp(template.name, "Master Playback Volume") &&
 		    chip->model->dac_tlv) {
 			template.tlv.p = chip->model->dac_tlv;