Metadata streams have priv_data set to NULL.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 0efb6106)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>