avformat/ivfenc: Change the length fields to 32 bits

There is no change in the encoded bitstream, but this
ensures that the written field length is consistent
with the reference implementation.

Unused bytes are zeroed out for backwards compatibility.
Signed-off-by: Raphaël Zumer <rzumer@tebako.net>

libavformat/ivfenc.c

@@ -84,7 +84,8 @@ static int ivf_write_trailer(AVFormatContext *s)
 
         avio_seek(pb, 24, SEEK_SET);
         // overwrite the "length" field (duration)
-        avio_wl64(pb, ctx->frame_cnt * ctx->sum_delta_pts / (ctx->frame_cnt - 1));
+        avio_wl32(pb, ctx->frame_cnt * ctx->sum_delta_pts / (ctx->frame_cnt - 1));
+        avio_wl32(pb, 0); // zero out unused bytes
         avio_seek(pb, end, SEEK_SET);
     }