bufio.UnreadRune: fix bug at EOF

Fixes #1132.

R=rsc
CC=golang-dev
https://golang.org/cl/2215045

src/pkg/bufio/bufio.go

@@ -207,6 +207,7 @@ func (b *Reader) ReadRune() (rune int, size int, err os.Error) {
 	for b.r+utf8.UTFMax > b.w && !utf8.FullRune(b.buf[b.r:b.w]) && b.err == nil {
 		b.fill()
 	}
+	b.lastRuneSize = -1
 	if b.r == b.w {
 		return 0, 0, b.err
 	}

src/pkg/bufio/bufio_test.go

@@ -319,6 +319,20 @@ func TestUnreadRuneError(t *testing.T) {
 	}
 }
 
+func TestUnreadRuneAtEOF(t *testing.T) {
+	// UnreadRune/ReadRune should error at EOF (was a bug; used to panic)
+	r := NewReader(strings.NewReader("x"))
+	r.ReadRune()
+	r.ReadRune()
+	r.UnreadRune()
+	_, _, err := r.ReadRune()
+	if err == nil {
+		t.Error("expected error at EOF")
+	} else if err != os.EOF {
+		t.Error("expected EOF; got", err)
+	}
+}
+
 func TestReadWriteRune(t *testing.T) {
 	const NRune = 1000
 	byteBuf := new(bytes.Buffer)