mov: Rework the check for invalid indexes in stsc

There are samples with invalid stsc that may work fine as is and
do not need extradata change. So ignore any out of range index, and
error out only when explode is set.

Found-by: Matthieu Bouron <matthieu.bouron@stupeflix.com>

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>