isl_basic_set_opt: accept NULL pointer on zero-size allocation
isl_basic_set_opt: accept NULL pointer on zero-size allocation

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

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