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

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

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