isl_mat_sub_alloc6: accept NULL pointer on zero-size allocation
isl_mat_sub_alloc6: accept NULL pointer on zero-size allocation

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

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