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

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

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