avcodec/ffv1enc: Try to choose slice count so that slice packet sizes are within the supported size

Fixes assertion failure
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffv1enc.c

@@ -848,10 +848,17 @@ FF_ENABLE_DEPRECATION_WARNINGS
     }
 
     if (s->version > 1) {
+        int plane_count = 1 + 2*s->chroma_planes + s->transparency;
         s->num_v_slices = (avctx->width > 352 || avctx->height > 288 || !avctx->slices) ? 2 : 1;
-        for (; s->num_v_slices < 9; s->num_v_slices++) {
+        for (; s->num_v_slices < 32; 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 || !avctx->slices)
+                int maxw = (avctx->width  + s->num_h_slices - 1) / s->num_h_slices;
+                int maxh = (avctx->height + s->num_v_slices - 1) / s->num_v_slices;
+                if (s->num_h_slices > avctx->width || s->num_v_slices > avctx->height)
+                    continue;
+                if (maxw * maxh * (int64_t)(s->bits_per_raw_sample+1) * plane_count > 8<<24)
+                    continue;
+                if (avctx->slices == s->num_h_slices * s->num_v_slices && avctx->slices <= MAX_SLICES || !avctx->slices)
                     goto slices_ok;
             }
         }