openpgp: Fix improper := shadowing

R=golang-dev, agl1, jnml
CC=golang-dev
https://golang.org/cl/4381058

src/pkg/crypto/openpgp/armor/encode.go

@@ -18,9 +18,9 @@ var armorEndOfLineOut = []byte("-----\n")
 // writeSlices writes its arguments to the given Writer.
 func writeSlices(out io.Writer, slices ...[]byte) (err os.Error) {
 	for _, s := range slices {
-		_, err := out.Write(s)
+		_, err = out.Write(s)
 		if err != nil {
-			return
+			return err
 		}
 	}
 	return