readelf: avoid division by zero on section entry size

ELF Tool Chain tickets #439, #444, #445, #467
Reported by: Alexander Cherepanov <cherepan@mccme.ru> (#467)
Reported by: antiAgainst (others)

Reviewed by: brooks
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D2338