isl_ast_graft.c: insert_pending_guard_nodes: avoid zero-size allocation
isl_ast_graft.c: insert_pending_guard_nodes: avoid zero-size allocation

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

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