The allocated size of sc->ctts_data is
(st->nb_index_entries + entries) * sizeof(*sc->ctts_data).

The size to memset at offset sc->ctts_data + sc->ctts_count should be
(st->nb_index_entries + entries - sc->ctts_count) *
sizeof(*sc->ctts_data))

The current code missed |entries| I believe, which was introduced in
https://patchwork.ffmpeg.org/patch/5541/.

However, after offline discussion, it seems the original code is much
more clear to read (before https://patchwork.ffmpeg.org/patch/5541/).

Hence this CL revert the memset logic to it's previous state by
remembering the |old_ctts_allocated_size|, and only memset the newly
allocated entries.

BUG=812567

Change-Id: Ibe94c7138e5818bfaae76866bfa6619a9b8a2b6b
Reviewed-on: https://chromium-review.googlesource.com/934925Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>