blake2b: fix constant size

Change-Id: Icae0ec427183d1e319f5a28d6372c823c269d898
Reviewed-on: https://go-review.googlesource.com/34252
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed