isl_input.c: obj_add: double check that add method is available
isl_input.c: obj_add: double check that add method is available

Until now, all object types have an add method, but the schedule
object we will be introducing next does not have a natural add
method and does not need one either since obj_add will never be
called on schedule objects.

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

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