isl_map.c: basic_set_maximal_difference_at: fix error handling
isl_map.c: basic_set_maximal_difference_at: fix error handling

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

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