string.cr: couple of review fixes in rabin-karp optimizations #19

Job	Run time
x86_64-gnu-test-preview_mt	5m 53s
x86_64-gnu-test (1.7.3)	33m 35s
x86_64-gnu-test (1.8.2)	31m 15s
x86_64-musl-test	39m 39s
x86_64-gnu-test (1.9.2)	36m 28s
x86_64-gnu-test (1.10.1)	33m 11s
check_format	1m 54s
x86_64-gnu-test (1.11.2)	33m 45s
x86_64-gnu-test (1.12.1)	31m 59s
deploy_api_docs	0s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 33s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	35m 33s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 23s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	33m 55s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 0s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	33m 40s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	34m 44s
	8h 10m 27s

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