ALSA: ac97: Fix possible NULL dereference in snd_ac97_mixer

smatch error:
sound/pci/ac97/ac97_codec.c:2354 snd_ac97_mixer() error:
we previously assumed 'rac97' could be null (see line 2072)

remove redundant assignment, return error if rac97 is NULL.

Fixes: da3cec35 ("ALSA: Kill snd_assert() in sound/pci/*")
Signed-off-by: Su Hui <suhui@nfschina.com>
Link: https://lore.kernel.org/r/20230615021732.1972194-1-suhui@nfschina.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/ac97/ac97_codec.c

@@ -2069,8 +2069,8 @@ int snd_ac97_mixer(struct snd_ac97_bus *bus, struct snd_ac97_template *template,
 		.dev_disconnect =	snd_ac97_dev_disconnect,
 	};
 
-	if (rac97)
-		*rac97 = NULL;
+	if (!rac97)
+		return -EINVAL;
 	if (snd_BUG_ON(!bus || !template))
 		return -EINVAL;
 	if (snd_BUG_ON(template->num >= 4))