Found by github.com/mvdan/unparam. Small performance win when the
utf8.RuneLen call is removed.

name               old time/op    new time/op    delta
AppendQuoteRune-4    21.7ns ± 0%    21.4ns ± 0%  -1.38%  (p=0.008 n=5+5)

Change-Id: Ieb3b3e1148db7a3d854c81555a491edeff549f43
Reviewed-on: https://go-review.googlesource.com/37831Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>