strings, bytes: ReadAt should not mutate receiver

CL 77580046 caused a data race issue with tests that assumes ReadAt
does not mutate receiver. This patch partially revert CL 77580046
to fix it.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77900043

src/pkg/bytes/reader.go

@@ -43,7 +43,6 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 }
 
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
-	r.prevRune = -1
 	if off < 0 {
 		return 0, errors.New("bytes: invalid offset")
 	}

src/pkg/bytes/reader_test.go

@@ -138,7 +138,6 @@ var UnreadRuneErrorTests = []struct {
 	f    func(*Reader)
 }{
 	{"Read", func(r *Reader) { r.Read([]byte{}) }},
-	{"ReadAt", func(r *Reader) { r.ReadAt([]byte{}, 0) }},
 	{"ReadByte", func(r *Reader) { r.ReadByte() }},
 	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
 	{"Seek", func(r *Reader) { r.Seek(0, 1) }},

src/pkg/strings/reader.go

@@ -42,7 +42,6 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 }
 
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
-	r.prevRune = -1
 	if off < 0 {
 		return 0, errors.New("strings: invalid offset")
 	}

src/pkg/strings/strings_test.go

@@ -863,7 +863,6 @@ var UnreadRuneErrorTests = []struct {
 	f    func(*Reader)
 }{
 	{"Read", func(r *Reader) { r.Read([]byte{}) }},
-	{"ReadAt", func(r *Reader) { r.ReadAt([]byte{}, 0) }},
 	{"ReadByte", func(r *Reader) { r.ReadByte() }},
 	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
 	{"Seek", func(r *Reader) { r.Seek(0, 1) }},