[autoboot] Cope properly with empty DHCP filenames

This (hopefully) fixes a regression introduced in commit e088892
("[autoboot] Connect SAN disk during a filename boot, if applicable").

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

 	 * filenames with and without initial slashes, which is
 	 * significant for TFTP.
 	 */
-	if ( ! uri_is_absolute ( uri ) ) {
+	if ( next_server.s_addr && filename[0] && ! uri_is_absolute ( uri ) ) {
 		uri_put ( uri );
 		snprintf ( buf, sizeof ( buf ), "tftp://%s/%s",
 			   inet_ntoa ( next_server ), filename );
-		uri = parse_uri ( filename );
+		uri = parse_uri ( buf );
 		if ( ! uri )
 			return NULL;
 	}
 	}
 
 	/* Treat empty URIs as absent */
-	if ( filename && ( ! filename->path ) )
+	if ( filename && ( ! uri_has_path ( filename ) ) )
 		filename = NULL;
 	if ( root_path && ( ! uri_is_absolute ( root_path ) ) )
 		root_path = NULL;