Removed IpAddress::IsValid(), added IpAddress::None

git-svn-id: https://sfml.svn.sourceforge.net/svnroot/sfml/branches/sfml2@1456 4e206d99-4929-0410-ac5d-dfc041789085

samples/ftp/Ftp.cpp

@@ -32,7 +32,7 @@ int main()
         std::cout << "Enter the FTP server address : ";
         std::cin  >> address;
     }
-    while (!address.IsValid());
+    while (address == sf::IpAddress::None);
 
     // Connect to the server
     sf::Ftp server;