unicode/bidi: remove vet complaint Change-Id: Ia179c919ecafbeeccb916abbcf553fc3a204dcfb Reviewed-on: https://go-review.googlesource.com/38360 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>