Calling tar.Reader.Read() used to work fine, but without this patch it panics.
Simply return EOF to indicate the tar.Reader.Next() needs to be called.

LGTM=iant, bradfitz
R=golang-codereviews, bradfitz, iant, mikioh.mikioh, dominik.honnef
CC=golang-codereviews
https://golang.org/cl/94530043