bytes: hoist error creation out of function

generating frame information in errors will cause this
function to no longer be inlined.

Updates #29934.

Change-Id: I1d7bc11707f1872d7315f627bfb9a12afa41e358
Reviewed-on: https://go-review.googlesource.com/c/161760
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/buffer.go

@@ -385,13 +385,15 @@ func (b *Buffer) UnreadRune() error {
 	return nil
 }
 
+var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
+
 // UnreadByte unreads the last byte returned by the most recent successful
 // read operation that read at least one byte. If a write has happened since
 // the last read, if the last read returned an error, or if the read read zero
 // bytes, UnreadByte returns an error.
 func (b *Buffer) UnreadByte() error {
 	if b.lastRead == opInvalid {
-		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
+		return errUnreadByte
 	}
 	b.lastRead = opInvalid
 	if b.off > 0 {