sound: Correctly check nvlist_unpack() error

The current check is never false and if nvlist_unpack() fails, we might
panic later down the road.

PR: 266144
Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D45237

(cherry picked from commit 64f4e2db6d19d8ab520903a197fcaa8cc7ab9f9a)
(cherry picked from commit 45feaa73c68011bbba647d1eb6f86a166a0453e9)

Approved by: re (cperciva)