isl_basic_map_plain_cmp: handle NULL arguments
isl_basic_map_plain_cmp: handle NULL arguments

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

Distrotech/isl,nicolasvasilache/isl,BenzoSM/isl,Meinersbur/isl,evaautomation/isl,UBERTC/isl,KangDroidSMProject/ISL,nicolasvasilache/isl,simbuerg/isl,Meinersbur/isl,Distrotech/isl,nicolasvasilache/isl,inducer/isl-mirror,abduld/isl,KangDroidSMProject/ISL,UBERTC/isl,abduld/isl,tobig/isl,tobig/isl,Meinersbur/isl,nicolasvasilache/isl,PollyLabs/isl,simbuerg/isl,simbuerg/isl,KangDroidSMProject/ISL,nicolasvasilache/isl,PollyLabs/isl,simbuerg/isl,KangDroidSMProject/ISL,Distrotech/isl,inducer/isl-mirror,PollyLabs/isl,tobig/isl,simbuerg/isl,evaautomation/isl,evaautomation/isl,UBERTC/isl,inducer/isl-mirror,UBERTC/isl,inducer/isl-mirror,PollyLabs/isl,Distrotech/isl,BenzoSM/isl,abduld/isl,tobig/isl,inducer/isl-mirror,BenzoSM/isl,Meinersbur/isl,PollyLabs/isl,BenzoSM/isl,BenzoSM/isl,KangDroidSMProject/ISL,abduld/isl,evaautomation/isl,Distrotech/isl