isl_ast_node_user_get_expr: check that input is indeed a user node
isl_ast_node_user_get_expr: check that input is indeed a user node

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

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