[tftp] Avoid setting current working URI to "tftp://0.0.0.0/"

Set the current working URI to NULL rather than to "tftp://0.0.0.0/".

Reported-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/tftp.c

 	 * applicators.
 	 */
 	if ( tftp_server.s_addr != last_tftp_server.s_addr ) {
-		snprintf ( uri_string, sizeof ( uri_string ),
-			   "tftp://%s/", inet_ntoa ( tftp_server ) );
-		uri = parse_uri ( uri_string );
-		if ( ! uri )
-			return -ENOMEM;
+		if ( tftp_server.s_addr ) {
+			snprintf ( uri_string, sizeof ( uri_string ),
+				   "tftp://%s/", inet_ntoa ( tftp_server ) );
+			uri = parse_uri ( uri_string );
+			if ( ! uri )
+				return -ENOMEM;
+		} else {
+			uri = NULL;
+		}
 		churi ( uri );
 		uri_put ( uri );
 	}