isl_tab_pip.c: tab_for_lexmin: accept NULL pointer on zero-size allocation
isl_tab_pip.c: tab_for_lexmin: accept NULL pointer on zero-size allocation

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

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