ALSA: ice1724 - Check for ac97 to avoid kernel oops

Cards with identical PCI ids but no AC97 config in EEPROM do not have
the ac97 field initialized. We must check for this case to avoid kernel oops.
Signed-off-by: NPavel Hofman <pavel.hofman@ivitera.com>
Cc: <stable@kernel.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/ice1712/amp.c

@@ -68,8 +68,11 @@ static int __devinit snd_vt1724_amp_init(struct snd_ice1712 *ice)
 
 static int __devinit snd_vt1724_amp_add_controls(struct snd_ice1712 *ice)
 {
-	/* we use pins 39 and 41 of the VT1616 for left and right read outputs */
-	snd_ac97_write_cache(ice->ac97, 0x5a, snd_ac97_read(ice->ac97, 0x5a) & ~0x8000);
+	if (ice->ac97)
+		/* we use pins 39 and 41 of the VT1616 for left and right
+		read outputs */
+		snd_ac97_write_cache(ice->ac97, 0x5a,
+			snd_ac97_read(ice->ac97, 0x5a) & ~0x8000);
 	return 0;
 }