[autoboot] Avoid using uri_dup() for constructed TFTP URI

uri_dup() chokes on duplicating a URI with a path that does not begin
with a slash.

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

src/usr/autoboot.c

  */
 static struct uri * parse_next_server_and_filename ( struct in_addr next_server,
 						     const char *filename ) {
+	char buf[ 23 /* "tftp://xxx.xxx.xxx.xxx/" */ + strlen ( filename )
+		  + 1 /* NUL */ ];
 	struct uri *uri;
-	struct uri *tmp;
 
 	/* Parse filename */
 	uri = parse_uri ( filename );
 	 * significant for TFTP.
 	 */
 	if ( ! uri_is_absolute ( uri ) ) {
-		tmp = uri;
-		tmp->scheme = "tftp";
-		tmp->host = inet_ntoa ( next_server );
-		uri = uri_dup ( tmp );
-		uri_put ( tmp );
+		uri_put ( uri );
+		snprintf ( buf, sizeof ( buf ), "tftp://%s/%s",
+			   inet_ntoa ( next_server ), filename );
+		uri = parse_uri ( filename );
 		if ( ! uri )
 			return NULL;
 	}