encoding/line: fix line returned after EOF

Fixes #1509.

R=r
CC=golang-dev
https://golang.org/cl/4167045

src/pkg/encoding/line/line.go

@@ -105,6 +105,9 @@ func (l *Reader) ReadLine() (line []byte, isPrefix bool, err os.Error) {
 		l.buf = l.buf[:oldLen+n]
 		if readErr != nil {
 			l.err = readErr
+			if len(l.buf) == 0 {
+				return nil, false, readErr
+			}
 		}
 	}
 	panic("unreachable")

src/pkg/encoding/line/line_test.go

@@ -7,6 +7,7 @@ package line
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
 	"os"
 	"testing"
 )
@@ -108,3 +109,25 @@ func TestReadAfterLines(t *testing.T) {
 		t.Errorf("bad result for Read: got %q; expected %q", outbuf.String(), restData)
 	}
 }
+
+func TestReadEmptyBuffer(t *testing.T) {
+	l := NewReader(bytes.NewBuffer(nil), 10)
+	line, isPrefix, err := l.ReadLine()
+	if err != os.EOF {
+		t.Errorf("expected EOF from ReadLine, got '%s' %t %s", line, isPrefix, err)
+	}
+}
+
+func TestLinesAfterRead(t *testing.T) {
+	l := NewReader(bytes.NewBuffer([]byte("foo")), 10)
+	_, err := ioutil.ReadAll(l)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	line, isPrefix, err := l.ReadLine()
+	if err != os.EOF {
+		t.Errorf("expected EOF from ReadLine, got '%s' %t %s", line, isPrefix, err)
+	}
+}