diracdec: check if reference could not be allocated

s->ref_pics[i] is later used as ref argument of interpolate_refplane,
where it is dereferenced.

If it is NULL, it causes a segmentation fault.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
