grep: handle malformed color environment values

[SichenLiang]

Ignore a malformed GREP_COLOR without printing the deprecation warning. Valid values still work and still produce the warning.
When parsing GREP_COLORS, stop at the first malformed entry or empty key, but keep entries parsed before it.

$ printf 'hit one\n' |
    env -u GREP_COLORS LC_ALL=C GREP_COLOR=4x2 \
    grep --color=always hit 2>&1 | cat -v

GNU prints the match in its default color without a warning. uutils previously printed a warning and used ^[[4x2m as the escape sequence.
Fixes #135

[codspeed]

Merging this PR will not alter performance

⚠️ 3 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• fixed_string
• search_pattern
• literal_no_match

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing SichenLiang:grep-malformed-colors (71fe9f6) with main (b68f81c)}

Footnotes

1. 17 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. ↩

[lhecker]

What's the point of trying to recover a malformed non-UTF8 GREP_COLORS env variable?

[SichenLiang]

Not much, GNU does keep valid entries around a bad byte, but this isn't a realistic environment value and isn't worth the extra handling. I dropped that part so a non-UTF-8 GREP_COLORS is ignored as a whole again.

[lhecker]

Thanks!