ffv1enc: use default number of slices if unspecified

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

libavcodec/ffv1.c

@@ -1106,7 +1106,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
     if(s->version>1){
         for(s->num_v_slices=2; s->num_v_slices<9; s->num_v_slices++){
             for(s->num_h_slices=s->num_v_slices; s->num_h_slices<2*s->num_v_slices; s->num_h_slices++){
-                if(avctx->slices == s->num_h_slices * s->num_v_slices && avctx->slices <= 64)
+                if(avctx->slices == s->num_h_slices * s->num_v_slices && avctx->slices <= 64 || !avctx->slices)
                     goto slices_ok;
             }
         }