elfdump: handle small files more gracefully

elfdump -E on an empty file would complain "Invalid argument" because
it tried to mmap zero bytes. With the -E flag, elfdump should
simply exit non-zero. For tiny files, the code would reference off
the end of the mapped region.

Ensure the file is large enough to contain an ELF header before mapping it.

MFC after: 1 week
Sponsored by: Dell EMC Isilon