debug/elf: perform stricter section header table checks in NewFile

If an ELF file has no section header table (shoff = 0), shnum must be
zero as well according to elf(5).

So far, when only shnum was zero but shoff was non-zero (i.e. in an
invalid ELF file) shstrndx wasn't properly checked and could result in
an 'index out of range' later on.

Fixes #10996

Change-Id: Ic248d2d77099b0036458e2a844b086a5f463c844
Reviewed-on: https://go-review.googlesource.com/c/162857


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>