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
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R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5604043