avcodec/huffyuvenc: only allocate stats_out when it will be used

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/frame_thread_encoder: restructure huffyuv checks

This makes them similar to the other checks

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/frame_thread_encoder: warn about huffyuv limitations

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