debug/elf: check for negative shoff and phoff fields

debug/elf: check for negative shoff and phoff fields

No test because we could add an infinite number of tests of bogus data.

For #47653
Fixes #52035

Change-Id: Iec7e2fe23f2dd1cf14bad2475422f243f51028f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/396880
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>