Correct ELF header parsing code to prevent invalid ELF sections from
disclosing memory.

Submitted by: markj
Reported by: Thomas Barabosch, Fraunhofer FKIE
Approved by: re (implicit)
Approved by: so
Security: FreeBSD-SA-18:12.elf
Security: CVE-2018-6924
Sponsored by: The FreeBSD Foundation