* Assert that the format is GNU.
Both GNU and STAR have some form of sparse file support with
incompatible header structures. Worse yet, both formats use the
'S' type flag to indicate the presence of a sparse file.
As such, we should check the format (based on magic numbers)
and fail early.

* Move realsize parsing logic into readOldGNUSparseMap.
This is related to the sparse parsing logic and belongs here.

* Fix the termination condition for parsing sparse fields.
The termination condition for reading the sparse fields
is to simply check if the first byte of the offset field is NULL.
This does not seem to be documented in the GNU manual, but this is
the check done by the both the GNU and BSD implementations:
	http://git.savannah.gnu.org/cgit/tar.git/tree/src/sparse.c?id=9a33077a7b7ad7d32815a21dee54eba63b38a81c#n731
	https://github.com/libarchive/libarchive/blob/1fa9c7bf90f0862036a99896b0501c381584451a/libarchive/archive_read_support_format_tar.c#L2207

* Fix the parsing of sparse fields to use parseNumeric.
This is what GNU and BSD do. The previous two links show that
GNU and BSD both handle base-256 and base-8.

* Detect truncated streams.
The call to io.ReadFull does not check if the error is io.EOF.
Getting io.EOF in this method is never okay and should always be
converted to io.ErrUnexpectedEOF.

* Simplify the function.
The logic is essentially a do-while loop so we can remove
some redundant code.

Change-Id: Ib2f601b1a283eaec1e41b1d3396d649c80749c4e
Reviewed-on: https://go-review.googlesource.com/28471Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>