isl_map_subtract.c: basic_map_collect_diff: avoid invalid access on error
isl_map_subtract.c: basic_map_collect_diff: avoid invalid access on error

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

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