net: don't return io.EOF from zero byte reads on Plan 9

Updates #15735.
Fixes #15741.

Change-Id: Ic4ad7e948e8c3ab5feffef89d7a37417f82722a1
Reviewed-on: https://go-review.googlesource.com/23199
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/fd_plan9.go

@@ -76,6 +76,9 @@ func (fd *netFD) Read(b []byte) (n int, err error) {
 		return 0, err
 	}
 	defer fd.readUnlock()
+	if len(b) == 0 {
+		return 0, nil
+	}
 	n, err = fd.data.Read(b)
 	if isHangup(err) {
 		err = io.EOF