io: fix nil Write bug in Pipe

R=nigeltao_golang
CC=golang-dev
https://golang.org/cl/194132

src/pkg/io/pipe.go

@@ -39,7 +39,7 @@ func (p *pipe) Read(data []byte) (n int, err os.Error) {
 		if !p.wclosed {
 			p.wpend = <-p.cr
 		}
-		if p.wpend == nil {
+		if p.wclosed {
 			return 0, p.werr
 		}
 		p.wtot = 0

src/pkg/io/pipe_test.go

@@ -236,3 +236,25 @@ func TestPipeWriteClose(t *testing.T) {
 		}
 	}
 }
+
+func TestWriteEmpty(t *testing.T) {
+	r, w := Pipe()
+	go func() {
+		w.Write([]byte{})
+		w.Close()
+	}()
+	var b [2]byte
+	ReadFull(r, b[0:2])
+	r.Close()
+}
+
+func TestWriteNil(t *testing.T) {
+	r, w := Pipe()
+	go func() {
+		w.Write(nil)
+		w.Close()
+	}()
+	var b [2]byte
+	ReadFull(r, b[0:2])
+	r.Close()
+}