crypto/ecdsa: fix case where p != 0 mod 8 and the hash length < p.
crypto/ecdsa: fix case where p != 0 mod 8 and the hash length < p.

I made a typo which breaks P-521.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6219057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go