// Unicode code point from the buffer.
// If no bytes are available, the error returned is io.EOF.
// If the bytes are an erroneous UTF-8 encoding, it
// consumes one byte and returns U+FFFD, 1.
func (b *Buffer) ReadRune() (r rune, size int, err error) {
	if b.empty() {
		// Buffer is empty, reset to recover space.
		b.Reset()
		return 0, 0, io.EOF
	}
	c := b.buf[b.off]
	if c < utf8.RuneSelf {