CVE-2023-53648

In the Linux kernel, the following vulnerability has been resolved:

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.