ffmpeg_opt: free incorrect priv_data for feed stream

new_output_stream creates a codec context with arbitraty picked codec.
Later data is updated, but priv_data are left alone.
There is a bit chance there is a mismatch between codecs.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>