mov: immediately return from mov_fix_index without old index entries

If there are no index entries, e_old = st->index_entries is only one
byte large, since it was created by av_realloc called with size 0.

Thus accessing e_old[0].timestamp causes a heap buffer overflow.

Reviewed-by: Sasi Inguva <isasi@google.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
