split: do not overflow on -n l/K when K is larger than the byte count

[AlejandroCoronadoN]

Problem

$ printf 'ab' > tiny
$ split -n l/5 tiny
thread 'main' panicked at src/uu/split/src/split.rs:1144:13: attempt to add with overflow

When there are more chunks than bytes, chunk_size_base = num_bytes / num_chunks
is 0 and the trailing empty chunks add 0 to num_bytes_should_be_written, so the
inner while num_bytes_should_be_written <= num_bytes_written loop never advances
and spins until chunk_number/skipped overflow.

Fix

Add chunk_number < num_chunks to the loop condition so it stops at the last
chunk. There are only num_chunks chunks, so this never truncates real output.

Verification

Compared against GNU split for -n l/5 on ab, -n l/3 on a, -n l/2 on
empty input, -n l/3 on a 10-byte line, and multi-line inputs: the file set and
contents match exactly (xaa gets the data, the rest are empty). Added a
regression test; the full test_split suite (129 tests) passes and cargo fmt
/ cargo clippy are clean.

[codspeed-hq]

Merging this PR will degrade performance by 7.83%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 73 untouched benchmarks
⏩ 318 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	df_with_path	573.7 µs	704.9 µs	-18.62%
⚡	Simulation	du_summarize_balanced_tree[(5, 4, 10)]	16.8 ms	16.1 ms	+4.38%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing AlejandroCoronadoN:fix-split-n-lk-overflow (eed3fdb) with main (822aa83)}

Footnotes

1. 318 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

I don't think we need 5 five comment lines