archive/tar: properly parse GNU base-256 encoding

archive/tar: properly parse GNU base-256 encoding

Motivation:
* Previous implementation did not detect integer overflow when
parsing a base-256 encoded field.
* Previous implementation did not treat the integer as a two's
complement value as specified by GNU.

The relevant GNU specification says:
<<<
GNU format uses two's-complement base-256 notation to store values
that do not fit into standard ustar range.
>>>

Fixes #12435

Change-Id: I4639bcffac8d12e1cb040b76bd05c9d7bc6c23a8
Reviewed-on: https://go-review.googlesource.com/17424
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>