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

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

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