The sparseFileReader is prone to two different forms of
denial-of-service attacks:
* A malicious tar file can cause an infinite loop
* A malicious tar file can cause arbitrary panics

This results because of poor error checking/handling, which this
CL fixes. While we are at it, add a plethora of unit tests to
test for possible malicious inputs.

Change-Id: I2f9446539d189f3c1738a1608b0ad4859c1be929
Reviewed-on: https://go-review.googlesource.com/15115Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>