net/SocketAddress: add CastTo()

src/net/ToString.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2011-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -85,7 +85,7 @@ ToString(SocketAddress address) noexcept
 #ifdef HAVE_UN
 	if (address.GetFamily() == AF_LOCAL)
 		/* return path of local socket */
-		return LocalAddressToString(*(const sockaddr_un *)address.GetAddress(),
+		return LocalAddressToString(address.CastTo<struct sockaddr_un>(),
 					    address.GetSize());
 #endif