sound: Do not check for NULL if sbuf is allocated with SBUF_AUTOEXTEND
Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D46846
(cherry picked from commit aaf84d0e96831cd886b8e1e4ae102b3ba703b90a)