internal/export/idna: fix error message

by moving incement to the right place.

Change-Id: I04d06639fde5bf6768b53ea33f347b3e145ade53
Reviewed-on: https://go-review.googlesource.com/38695
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed