cdg: Forward error from avio_size() in read_header() function

Signed-off-by: Diego Biurrun <diego@biurrun.de>
Merge commit '44386aaad870cbd80ae0d08247ebc663476446ff'

* commit '44386aaad870cbd80ae0d08247ebc663476446ff':
  cdg: Forward error from avio_size() in read_header() function

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/cdg: Do not fail if filesize cannot be determined

This fixes cdg with piped input which was broken by the previous commit

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>