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

openpgp/clearsign/clearsign.go

@@ -296,10 +296,10 @@ func Encode(w io.Writer, privateKey *packet.PrivateKey, config *packet.Config) (
 		return nil, errors.UnsupportedError("unknown hash type: " + strconv.Itoa(int(hashType)))
 	}
 
-	h := hashType.New()
-	if h == nil {
+	if !hashType.Available() {
 		return nil, errors.UnsupportedError("unsupported hash type: " + strconv.Itoa(int(hashType)))
 	}
+	h := hashType.New()
 
 	buffered := bufio.NewWriter(w)
 	// start has a \n at the beginning that we don't want here.