erp5_web: base url does't always start with root url

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/Base_convertHtmlToSingleFile.py

@@ -252,8 +252,10 @@ else:
   root_url = base_url_root_object.absolute_url()
   base_url = base_url_object.absolute_url()
 
-assert base_url_object.getRelativeUrl().startswith(base_url_root_object.getRelativeUrl())
-base_path = base_url_object.getRelativeUrl()[len(base_url_root_object.getRelativeUrl()):]
+if base_url_object.getRelativeUrl().startswith(base_url_root_object.getRelativeUrl()):
+  base_path = base_url_object.getRelativeUrl()[len(base_url_root_object.getRelativeUrl()):]
+else:
+  base_path = base_url_object.getRelativeUrl()
 if not base_path.startswith("/"):
   base_path = "/" + base_path