Peter Zhu 50bf437341 Fix String#sub for GC compaction
The test fails when RGENGC_CHECK_MODE is turned on:

    TestString#test_sub_gc_compact_stress = 9.42 s
    1) Failure:
    TestString#test_sub_gc_compact_stress [test/ruby/test_string.rb:2089]:
    <"aaa [amp] yyy"> expected but was
    <"aaa [] yyy">.
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