ctts data in ffmpeg relies on the index entries array to be 1:1
with samples... yet sc->sample_count can be read directly from
the 'stsz' box and index entries are only generated if a chunk
count has been read from 'stco' box.

Ensure that if sc->sample_count > 0, sc->chunk_count is too as
a basic sanity check. Additionally we need to check that after
the index is built we have the right number of entries, so we
also check in mov_read_trun() that sc->sample_count ==
st->nb_index_entries.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>