Fix an off by one in `rb_ary_resize` #616

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 13s
make (check, i686)	9m 44s
make (check, --disable-yjit)	6m 44s
make (check, --enable-shared --enable-load-relative)	7m 4s
make (check, true, --with-shared-gc=/home/runner/ruby_gc)	7m 12s
make (test-bundler-parallel, 50)	27m 6s
make (test-bundled-gems)	17m 4s
make (check, ubuntu-20.04)	9m 16s
make (check, ubuntu-24.04)	9m 44s
Ubuntu result	1s
	1h 42m 8s

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