isl_polynomial.c: with_merged_divs: accept NULL pointer on zero-size allocation
isl_polynomial.c: with_merged_divs: accept NULL pointer on zero-size allocation

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

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