crypto/openpgp: fixed dangerous use of for loop variable

In function readSignedMessage a pointer to for loop variable 'key' was incorrectly being assigned
to md.SignedBy. Changed so that md.SignedBy is pointing to the 'more correct' memory position.

R=golang-dev, r, agl
CC=golang-dev
https://golang.org/cl/4631088

src/pkg/crypto/openpgp/read.go

@@ -250,11 +250,12 @@ FindLiteralData:
 			md.IsSigned = true
 			md.SignedByKeyId = p.KeyId
 			keys := keyring.KeysById(p.KeyId)
-			for _, key := range keys {
+			for i, key := range keys {
 				if key.SelfSignature.FlagsValid && !key.SelfSignature.FlagSign {
 					continue
 				}
-				md.SignedBy = &key
+				md.SignedBy = &keys[i]
+				break
 			}
 		case *packet.LiteralData:
 			md.LiteralData = p