sound: Fix memory leak in chn_init()

In case chn_init() fails, we are leaking memory by not freeing the
feeder.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45979

(cherry picked from commit 16329b7b3276f09dc70bdd1ca8bc605546fe71e0)