[uri] Special case NULL in churi()

resolve_uri() doesn't (and probably shouldn't) handle NULL relative_uri.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/cwuri.c

  * @v uri		New working URI, or NULL
  */
 void churi ( struct uri *uri ) {
-	struct uri *new_uri;
+	struct uri *new_uri = NULL;
+
+	if ( uri )
+		new_uri = resolve_uri ( cwuri, uri );
 
-	new_uri = resolve_uri ( cwuri, uri );
 	uri_put ( cwuri );
 	cwuri = new_uri;
 }