isl_tab.c: restore_basis: accept NULL pointer on zero-size allocation
isl_tab.c: restore_basis: accept NULL pointer on zero-size allocation

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

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