* apt-pkg/contrib/fileutl.cc:

* apt-pkg/contrib/fileutl.cc:
  - Add a FileFd::FileSize() method to get the size of the underlying
    file and not the size of the content in the file as FileFd::Size()
    does - the sizes can differ since the direct gzip integration
* methods/{gzip,bzip2}.cc:
  - use FileSize() to determine if the file is invalid (Closes: #600852)