allowing access to the size but not the extradata itself is not useful
and could lead to potential problems if writing happens through this field
Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Reviewed-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>