Misc Coverity fixes in xnb(4)

Most of these are null pointer dereferences or missing error checks in the
unit tests. One is a missing error check in xnb_attach_failed. None can
cause real problems in running systems.

Reported by: Coverity
CIDs: 1092469 1092468 1092467 2092466 1092465 1092512 1092511 1092510
CIDs: 1092510 1092509 1092508 1092507
Reviewed by: royger
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9234