isl_transitive_closure.c: union_components: avoid zero-size allocation
isl_transitive_closure.c: union_components: avoid zero-size allocation

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

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