go.crypto/openpgp: fix hash presence checks.

At some point in the distant past, crypto.Hash.New() changed from
returning nil when a hash function wasn't provided, to panicing. Some
of the code in openpgp predates this and was still using the nil check.

LGTM=bradfitz
R=sburford, bradfitz
CC=golang-codereviews
https://golang.org/cl/104850045
5 files changed