blake2b: fix constant size
blake2b: fix constant size

Change-Id: Icae0ec427183d1e319f5a28d6372c823c269d898
Reviewed-on: https://go-review.googlesource.com/34252
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,ScriptRock/crypto,golang/crypto,golang/crypto,ScriptRock/crypto