Set string length in string_spec_RSTRING_PTR_read #2339

Job	Run time
RuboCop	44s
specs (macos, 3.3.10)	1m 23s
specs (macos, 4.0.1)	1m 37s
specs (ubuntu, 4.0.1, LC_ALL=C)	1m 35s
specs (windows, 3.3.10)	3m 47s
specs (ubuntu, 4.0.1)	1m 32s
specs (macos, 3.4.8)	1m 35s
specs (windows, 3.4.8)	3m 39s
specs (windows, 4.0.1)	3m 33s
specs (ubuntu, 3.4.8)	1m 41s
specs (ubuntu, 4.0.1, --repeat 2)	2m 10s
specs (ubuntu, 3.3.10)	1m 30s
specs (ubuntu, 4.0.1, --parser=parse.y)	1m 35s
specs (ubuntu, 4.0.1, --enable-frozen-string-literal)	1m 29s
	27m 50s

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