split: fix bug with large arguments to -C #5806

Job	Run time
Build the fuzzers	4m 1s
Run the fuzzers (fuzz_test, true)	4m 2s
Run the fuzzers (fuzz_date, false)	3m 58s
Run the fuzzers (fuzz_expr, true)	4m 5s
Run the fuzzers (fuzz_printf, false)	4m 3s
Run the fuzzers (fuzz_echo, true)	4m 2s
Run the fuzzers (fuzz_seq, false)	3m 7s
Run the fuzzers (fuzz_sort, false)	3m 17s
Run the fuzzers (fuzz_wc, false)	3m 11s
Run the fuzzers (fuzz_cut, false)	3m 14s
Run the fuzzers (fuzz_split, false)	3m 10s
Run the fuzzers (fuzz_tr, false)	3m 12s
Run the fuzzers (fuzz_env, false)	3m 0s
Run the fuzzers (fuzz_cksum, false)	3m 14s
Run the fuzzers (fuzz_parse_glob, true)	3m 56s
Run the fuzzers (fuzz_parse_size, true)	3m 53s
Run the fuzzers (fuzz_parse_time, true)	3m 52s
Run the fuzzers (fuzz_seq_parse_number, true)	3m 56s
	1h 5m 13s

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