ALSA: oxygen: prevent muting of nonexistent AC97 controls

The Xonar DX does not have CD Capture controls, so we have to check that
a control actually exists before muting it.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/oxygen/oxygen_mixer.c

@@ -512,9 +512,12 @@ static int ac97_switch_get(struct snd_kcontrol *ctl,
 
 static void mute_ac97_ctl(struct oxygen *chip, unsigned int control)
 {
-	unsigned int priv_idx = chip->controls[control]->private_value & 0xff;
+	unsigned int priv_idx;
 	u16 value;
 
+	if (!chip->controls[control])
+		return;
+	priv_idx = chip->controls[control]->private_value & 0xff;
 	value = oxygen_read_ac97(chip, 0, priv_idx);
 	if (!(value & 0x8000)) {
 		oxygen_write_ac97(chip, 0, priv_idx, value | 0x8000);