Update services/s3.class.php

I've changed the get_object_filesize() method.  Originally, the method gets the object.  If the object doesn't exists, it returns 0.
This is problematic in applications I've used the method in.  Getting a filesize of 0 implies that a file was written with 0 bytes (Usually indicating a failure of some kind when creating the original object, or a file that is in progress of being uploaded or created).
Instead, I propose returning -1 for an object that doesn't exist to distinguish a missing file from a 0 byte (corrupted) file.
I also added a check for filesize = -1 to the "friendly_format" if statement to bypass the size_readable method when a file doesn't exists.