avcodec/ffv1enc: Fix size of first slice

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit cff1c0ed

)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ffv1enc.c

@@ -1089,7 +1089,6 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     FFV1Context *f      = avctx->priv_data;
     RangeCoder *const c = &f->slice_context[0]->c;
     AVFrame *const p    = f->picture.f;
-    int used_count      = 0;
     uint8_t keystate    = 128;
     uint8_t *buf_p;
     int i, ret;
@@ -1178,11 +1177,17 @@ FF_ENABLE_DEPRECATION_WARNINGS
         }
     }
 
-    for (i = 1; i < f->slice_count; i++) {
+    for (i = 0; i < f->slice_count; i++) {
         FFV1Context *fs = f->slice_context[i];
-        uint8_t *start  = pkt->data + (pkt->size - used_count) * (int64_t)i / f->slice_count;
+        uint8_t *start  = pkt->data + pkt->size * (int64_t)i / f->slice_count;
         int len         = pkt->size / f->slice_count;
-        ff_init_range_encoder(&fs->c, start, len);
+        if (i) {
+            ff_init_range_encoder(&fs->c, start, len);
+        } else {
+            av_assert0(fs->c.bytestream_end >= fs->c.bytestream_start + len);
+            av_assert0(fs->c.bytestream < fs->c.bytestream_start + len);
+            fs->c.bytestream_end = fs->c.bytestream_start + len;
+        }
     }
     avctx->execute(avctx, encode_slice, &f->slice_context[0], NULL,
                    f->slice_count, sizeof(void *));