[ALSA] Fix NULL pointer access

MIXART driver
Fixed NULL pointer access when id string isn't given.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/mixart/mixart.c

@@ -1352,7 +1352,7 @@ static int __devinit snd_mixart_probe(struct pci_dev *pci,
 			idx = index[dev];
 		else
 			idx = index[dev] + i;
-		snprintf(tmpid, sizeof(tmpid), "%s-%d", id[dev], i);
+		snprintf(tmpid, sizeof(tmpid), "%s-%d", id[dev] ? id[dev] : "MIXART", i);
 		card = snd_card_new(idx, tmpid, THIS_MODULE, 0);
 
 		if (! card) {