isl_vertices.c: vertices_from_list: accept NULL pointer on zero-size allocation
isl_vertices.c: vertices_from_list: accept NULL pointer on zero-size allocation

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

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