dvbsubdec: fix division by zero in compute_default_clut

This problem was introduced in commit
4b90dcb8493552c17a811c8b1e6538dae4061f9d.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
