bugfix in calibre's "input.py"

def resource_adder(self, link_, base=None):
        link = self.urlnormalize(link_)
        link, frag = self.urldefrag(link)
        link = unquote(link).replace('/', os.sep)
        self.oeb.log.debug('Adding', link)
        if not link.strip():
            return link_
        try:
            if base and not os.path.isabs(link):
                link = os.path.join(base, link)
            link = os.path.abspath(link)
            # PypeBros: how about URLs that have non-ASCII characters ?
        except:
            return link_
        if not os.access(link, os.R_OK):
            return link_
        if os.path.isdir(link):
            self.log.warn(link_, 'is a link to a directory. Ignoring.')
            return link_