ALSA: ac97 - Fix missing NULL check in snd_ac97_cvol_new()

commit 733a48e5ae5bf28b046fad984d458c747cbb8c21 upstream.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=44721

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>