isl_scheduler.c: node_extract_partial_schedule_multi_aff: handle NULL input
isl_scheduler.c: node_extract_partial_schedule_multi_aff: handle NULL input

This function is currently always called with a non-NULL input,
but after the next commit, it may get called with a NULL input
in exceptional cases.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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