isl_test.c: before_for: return early on error
isl_test.c: before_for: return early on error

In particular, don't print any spurious error messages.

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

inducer/isl-mirror,BenzoSM/isl,nicolasvasilache/isl,pierrotdelalune/isl,pierrotdelalune/isl,PollyLabs/isl,nicolasvasilache/isl,cfx-next/toolchain_isl-upstream,tobig/isl,tobig/isl,serge-sans-paille/isl,PollyLabs/isl,KangDroidSMProject/ISL,BobSaget-Mod/libisl,KangDroidSMProject/ISL,inducer/isl-mirror,simbuerg/isl,simbuerg/isl,Meinersbur/isl,crossbuild/isl,Distrotech/isl,BenzoSM/isl,simbuerg/isl,evaautomation/isl,BenzoSM/isl,VanirLLVM/toolchain_isl,abduld/isl,VanirLLVM/toolchain_isl,inducer/isl-mirror,pierrotdelalune/isl,evaautomation/isl,nicolasvasilache/isl,UBERTC/isl,jleben/isl,evaautomation/isl,Distrotech/isl,VanirLLVM/toolchain_isl,abduld/isl,jleben/isl,BenzoSM/isl,UBERTC/isl,cfx-next/toolchain_isl-upstream,VanirLLVM/toolchain_isl,jleben/isl,PollyLabs/isl,Distrotech/isl,jleben/isl,epowers/isl,evaautomation/isl,abduld/isl,epowers/isl,cfx-next/toolchain_isl-upstream,KangDroidSMProject/ISL,PollyLabs/isl,crossbuild/isl,serge-sans-paille/isl,simbuerg/isl,crossbuild/isl,epowers/isl,Meinersbur/isl,Distrotech/isl,cfx-next/toolchain_isl-upstream,serge-sans-paille/isl,epowers/isl,crossbuild/isl,tobig/isl,UBERTC/isl,SaberMod/isl-current,tobig/isl,BobSaget-Mod/libisl,Meinersbur/isl,pierrotdelalune/isl,nicolasvasilache/isl,KangDroidSMProject/ISL,pierrotdelalune/isl,abduld/isl,epowers/isl,simbuerg/isl,BobSaget-Mod/libisl,serge-sans-paille/isl,SaberMod/isl-current,Distrotech/isl,KangDroidSMProject/ISL,inducer/isl-mirror,jleben/isl,serge-sans-paille/isl,SaberMod/isl-current,BobSaget-Mod/libisl,inducer/isl-mirror,PollyLabs/isl,BenzoSM/isl,SaberMod/isl-current,BobSaget-Mod/libisl,cfx-next/toolchain_isl-upstream,Meinersbur/isl,nicolasvasilache/isl,VanirLLVM/toolchain_isl,UBERTC/isl