avcodec/ffv1enc: fix bps for >8bit yuv when not explicitly set

Fixes Ticket4636
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ffv1enc.c

@@ -753,7 +753,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
         s->chroma_planes = desc->nb_components < 3 ? 0 : 1;
         s->colorspace = 0;
         s->transparency = desc->nb_components == 4;
-        if (!avctx->bits_per_raw_sample)
+        if (!avctx->bits_per_raw_sample && !s->bits_per_raw_sample)
             s->bits_per_raw_sample = 8;
         break;
     case AV_PIX_FMT_RGB32: