encoder: bytes.Buffer will never error on Write*()

This is explicitly stated in bytes.Buffer documentation and is already
relyed by ogórek in another place:

https://github.com/kisielk/og-rek/blob/c7dbf2e4/ogorek.go#L524

ogorek.go

@@ -531,10 +531,7 @@ func (d *Decoder) loadUnicode() error {
 		if err != nil {
 			return err
 		}
-		_, err = buf.WriteRune(r)
-		if err != nil {
-			return err
-		}
+		buf.WriteRune(r)
 	}
 	if len(sline) > 0 {
 		return fmt.Errorf("characters remaining after loadUnicode operation: %s", sline)