sched: fix infinite loop if mblks_rem=0 initially

[two-heart]

as already noted by the TODO comment, we must handle the mblks_rem zero case

[github-actions]

Performance Measurements ⏳

Suite	Baseline	New	Change
backtest mainnet-368528500-perf per slot	0.063709 s	0.063868 s	0.250% ✅
backtest mainnet-368528500-perf snapshot load	2.196 s	2.15 s	-2.095% ✅
backtest mainnet-368528500-perf total elapsed	63.709257 s	63.868235 s	0.250% ✅
firedancer mem usage with mainnet.toml	974.13 GiB	974.13 GiB	0.000% ✅