isl_schedule.c: sort_band_list: take into account that parent may be NULL
isl_schedule.c: sort_band_list: take into account that parent may be NULL

We missed this in 81bf4c2 (isl_schedule_get_band_forest: sort bands
in band list, Tue Feb 26 12:35:16 2013 +0100).

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

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