bsnmpwalk: Fix crash on invalid data

PR: 258570
Reported by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: emaste, markj
Differential Revision: https://reviews.freebsd.org/D48422

(cherry picked from commit f021e3573519ff192fc708cda9ca4bba264c96f7)
(cherry picked from commit 7fbbab2d326bcbfa1592c6104b745cd5973f5aaa)