cr_bsd_visible(): Style fixes

Explicitly test for non-zero return codes.

Separate assignment and testing of 'error' in distinct lines.

Reviewed by: emaste, kib
Approved by: emaste (mentor)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43335

(cherry picked from commit a1e37beb45d270ae9926eff3b93e9398d08c94e5)