bus: Return 0 if reading an ivar fails

In the non-INVARIANTS case, return 0 rather than stack garbage if
reading an ivar fails (in the INVARIANTS case, we still panic).

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D54078

(cherry picked from commit 4c2295c15860e70c8bd3f05f9229d2fc40dfd50d)