Partial base_path match

The compressed CSS and JS of our sites is stored on S3 and advagg is helping us to replace relative paths in CSS with absolute ones.

For one of our sub sites, we use a base path '/es/' which is accidentally matched with 'sites/all/modules/...', so all paths become invalid as it drops 'sites'. Because of the 'es/' within 'sites/', it assumes base_path is present. Below is patch with an attempted fix which seems to work for us. But it needs wider testing as I do not really understand the real purpose why it's there in the first place.