Fix an off by one in `rb_ary_resize` #520

Job	Run time
cargo test	49s
cargo clippy	28s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-12 --enable-yjit=dev)	4m 6s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	8m 24s
make (check, --enable-yjit=dev)	11m 41s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	15m 52s
make (test-bundled-gems, --enable-yjit=dev)	20m 17s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	6m 4s
YJIT Ubuntu result	1s
	1h 7m 42s

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