isl_tab_sample: treat emptiness of tableau as error
isl_tab_sample: treat emptiness of tableau as error

We only modify the tableau through calls to isl_tab_add_valid_eq,
which should never result in an empty tableau, given that
the tableau was initially not empty.

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

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