ALSA: ac97: Fix possible error value of *rac97

Before committing 79597c8, *rac97 always be NULL if there is
an error. When error happens, make sure *rac97 is NULL is safer.

For examble, in snd_vortex_mixer():
	err = snd_ac97_mixer(pbus, &ac97, &vortex->codec);
	vortex->isquad = ((vortex->codec == NULL) ?
		0 : (vortex->codec->ext_id&0x80));
If error happened but vortex->codec isn't NULL, this may cause some
problems.

Move the judgement order to be clearer and better.

Fixes: 79597c8 ("ALSA: ac97: Fix possible NULL dereference in snd_ac97_mixer")
Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Su Hui <suhui@nfschina.com>
Link: https://lore.kernel.org/r/20230823025212.1000961-1-suhui@nfschina.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: Su Hui

sound/pci/ac97/ac97_codec.c

@@ -2069,10 +2069,9 @@ int snd_ac97_mixer(struct snd_ac97_bus *bus, struct snd_ac97_template *template,
 		.dev_disconnect =	snd_ac97_dev_disconnect,
 	};
 
-	if (!rac97)
-		return -EINVAL;
-	if (snd_BUG_ON(!bus || !template))
+	if (snd_BUG_ON(!bus || !template || !rac97))
 		return -EINVAL;
+	*rac97 = NULL;
 	if (snd_BUG_ON(template->num >= 4))
 		return -EINVAL;
 	if (bus->codec[template->num])