crypto/openpgp/packet: fix private key checksum

I misinterpreted http://tools.ietf.org/html/rfc4880#section-5.5.3
and implemented the sum of 16-bit values, rather than the 16-bit sum
of 8-bit values.

Thanks to Szabolcs Nagy for pointing it out.

R=bradfitz, r, rsc
CC=golang-dev
https://golang.org/cl/5372091

src/pkg/crypto/openpgp/packet/private_key.go

@@ -99,13 +99,9 @@ func (pk *PrivateKey) parse(r io.Reader) (err error) {
 }
 
 func mod64kHash(d []byte) uint16 {
-	h := uint16(0)
-	for i := 0; i < len(d); i += 2 {
-		v := uint16(d[i]) << 8
-		if i+1 < len(d) {
-			v += uint16(d[i+1])
-		}
-		h += v
+	var h uint16
+	for _, b := range d {
+		h += uint16(b)
 	}
 	return h
 }