ALSA: opti93x - Fix NULL dereference

Probing non-existing device causes Oops with snd-opti93x driver
due to NULL access in the destructor of the error path.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Rene Herman <rene.herman@gmail.com>
Acked-by: Rene Herman <rene.herman@gmail.com>
Tested-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Ingo Molnar <mingo@elte.hu>

sound/isa/opti9xx/opti92x-ad1848.c

@@ -688,7 +688,7 @@ static void snd_card_opti9xx_free(struct snd_card *card)
 	if (chip) {
 #ifdef OPTi93X
 		struct snd_cs4231 *codec = chip->codec;
-		if (codec->irq > 0) {
+		if (codec && codec->irq > 0) {
 			disable_irq(codec->irq);
 			free_irq(codec->irq, codec);
 		}