isl_ast_codegen.c: foreach_iteration: improve error handling
isl_ast_codegen.c: foreach_iteration: improve error handling

In particular, do not rely on the value of "n" when
find_unroll_lower_bound fails.

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

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