isl_sample.c: drop_constraints_involving: avoid NULL pointer dereference
isl_sample.c: drop_constraints_involving: avoid NULL pointer dereference

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