archive/zip: use bufio.Reset rather than NewReader

A clean up for CL 408734, suggested by Joe Tsai.

Change-Id: Ida9db0b8d31785d5640938c286c9c6c82c27f457
Reviewed-on: https://go-review.googlesource.com/c/go/+/410154
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>

Author: ianlancetaylor

src/archive/zip/reader.go

@@ -133,7 +133,7 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 			if _, err = rs.Seek(int64(end.directoryOffset), io.SeekStart); err != nil {
 				return err
 			}
-			buf = bufio.NewReader(rs)
+			buf.Reset(rs)
 			continue
 		}