MFC r314148, r314150

MFC r314148, r314150

r314148:
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
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D9234

r314150:
Fix the xnb(4) unit tests

One test was inadvertently expecting a bug in the kernel's sscanf
implementation circa 2012. I don't know when that bug got fixed.

Reported by:	royger
Reviewed by:	royger
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D9766

git-svn-id: svn://svn.freebsd.org/base/stable/10@319222 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f