isl_scheduler.c: add_inter_validity_constraints: avoid NULL pointer dereference
isl_scheduler.c: add_inter_validity_constraints: avoid NULL pointer dereference

The actual potential NULL pointer dereference is further down
in the function, but it easier to check before anything has been
allocated.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,nicolasvasilache/isl,inducer/isl-mirror,Meinersbur/isl,PollyLabs/isl,tobig/isl,inducer/isl-mirror,tobig/isl,nicolasvasilache/isl,PollyLabs/isl,nicolasvasilache/isl,Meinersbur/isl,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,inducer/isl-mirror,tobig/isl,PollyLabs/isl,Meinersbur/isl,tobig/isl