archive/tar: make Reader.Read errors persistent

If the stream is in an inconsistent state, it does not make sense
that Reader.Read can be called and possibly succeed.

Change-Id: I9d1c5a1300b2c2b45232188aa7999e350809dcf2
Reviewed-on: https://go-review.googlesource.com/15177Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/tar/reader.go

@@ -763,9 +763,13 @@ func (tr *Reader) numBytes() int64 {
 // It returns 0, io.EOF when it reaches the end of that entry,
 // until Next is called to advance to the next entry.
 func (tr *Reader) Read(b []byte) (n int, err error) {
+	if tr.err != nil {
+		return 0, tr.err
+	}
 	if tr.curr == nil {
 		return 0, io.EOF
 	}
+
 	n, err = tr.curr.Read(b)
 	if err != nil && err != io.EOF {
 		tr.err = err