etdump: exit on error if section header or entry offset is OOB

PR: 263663
Reported by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35868

(cherry picked from commit ddf77ec392717e7eaf278c0f201b561afac97b87)