Prior to Go1.8, the Writer had a bug where it would output
an invalid tar file in certain rare situations because the logic
incorrectly believed that the old GNU format had a prefix field.
This is wrong and leads to an output file that mangles the
atime and ctime fields, which are often left unused.

In order to continue reading tar files created by former, buggy
versions of Go, we skeptically parse the atime and ctime fields.
If we are unable to parse them and the prefix field looks like
an ASCII string, then we fallback on the pre-Go1.8 behavior
of treating these fields as the USTAR prefix field.

Note that this will not use the fallback logic for all possible
files generated by a pre-Go1.8 toolchain. If the generated file
happened to have a prefix field that parses as valid
atime and ctime fields (e.g., when they are valid octal strings),
then it is impossible to distinguish between an valid GNU file
and an invalid pre-Go1.8 file.

Fixes #21005

Change-Id: Iebf5c67c08e0e46da6ee41a2e8b339f84030dd90
Reviewed-on: https://go-review.googlesource.com/53635
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>