kldxref: Tidy error handling for invalid addresses

• Don't print an offset value that's always zero.

• ef_get_offset can't return -1.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43124

(cherry picked from commit 6d46e2e0a7f345459fb42e666c7cb2e987c70857)