net/IPv6Address: pass reference to Cast()

src/net/IPv6Address.hxx

@@ -124,7 +124,7 @@ public:
 	 * Return a downcasted reference to the address.  This call is
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */
-	static constexpr const IPv6Address &Cast(const SocketAddress src) noexcept {
+	static constexpr const IPv6Address &Cast(const SocketAddress &src) noexcept {
 		/* this reinterpret_cast works because this class is
 		   just a wrapper for struct sockaddr_in6 */
 		return *(const IPv6Address *)(const void *)src.GetAddress();