numfmt: protect name from argv[0], perf +3%

[oech3]

Strange perf gain #11378 (comment)

[codspeed-hq]

Merging this PR will improve performance by 3.38%

⚡ 1 improved benchmark
✅ 297 untouched benchmarks
⏩ 48 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	numfmt_padding[(10000, 50)]	49.5 ms	47.9 ms	+3.38%

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_{Comparing oech3:numfmt-name-perf (41f3714) with main (2cb9b88)}

Footnotes

1. 48 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. ↩

[cakebaker]

Thanks!