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 not alter performance

✅ 353 untouched benchmarks
⏩ 50 skipped benchmarks¹

---

_{Comparing AlejandroCoronadoN:fix-split-n-lk-overflow (f4b23f4) with main (dc784af)}

Footnotes

1. 50 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

[AlejandroCoronadoN]

Shortened the comment to two lines. Thanks!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/sparse-to-pipe. tests/cp/sparse-to-pipe is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.