Use cloneSlice in unpackDataA and unpackDataAAAA

I missed this pattern in #1432. These seem to be the only two
occurrences.

Updates #1432

msg_helpers.go

@@ -20,9 +20,7 @@ func unpackDataA(msg []byte, off int) (net.IP, int, error) {
 	if off+net.IPv4len > len(msg) {
 		return nil, len(msg), &Error{err: "overflow unpacking a"}
 	}
-	a := append(make(net.IP, 0, net.IPv4len), msg[off:off+net.IPv4len]...)
-	off += net.IPv4len
-	return a, off, nil
+	return cloneSlice(msg[off : off+net.IPv4len]), off + net.IPv4len, nil
 }
 
 func packDataA(a net.IP, msg []byte, off int) (int, error) {
@@ -47,9 +45,7 @@ func unpackDataAAAA(msg []byte, off int) (net.IP, int, error) {
 	if off+net.IPv6len > len(msg) {
 		return nil, len(msg), &Error{err: "overflow unpacking aaaa"}
 	}
-	aaaa := append(make(net.IP, 0, net.IPv6len), msg[off:off+net.IPv6len]...)
-	off += net.IPv6len
-	return aaaa, off, nil
+	return cloneSlice(msg[off : off+net.IPv6len]), off + net.IPv6len, nil
 }
 
 func packDataAAAA(aaaa net.IP, msg []byte, off int) (int, error) {