Metadata streams have priv_data set to NULL.
Reviewed-by: Josh de Kock <josh@itanimul.li>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit fdb8c455)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>