Reduce risk of uninit ac in luma_ac and encode_coeffs #4704

Job	Run time
rustfmt-clippy	2m 55s
build-windows (cargo-build, x86_64-pc-windows-msvc)	13m 45s
build-macos (Cargo build (x64), cargo-build, x86_64-apple-darwin)	2m 47s
build-windows (cargo-test, x86_64-pc-windows-msvc)	13m 41s
build-unix (beta-build)	4m 38s
build-macos (Cargo test (x64), cargo-test, x86_64-apple-darwin)	8m 24s
build-windows (cargo-c, x86_64-pc-windows-gnu)	14m 12s
build-unix (1.70.0-tests)	8m 43s
build-macos (Cargo C-build (x64), cargo-c, x86_64-apple-darwin)	7m 2s
build-unix (aom-tests)	11m 31s
build-macos (Cargo build (Arm64), cargo-build, aarch64-apple-darwin)	5m 41s
build-unix (dav1d-tests)	10m 8s
build-macos (Cargo C-build (Arm64), cargo-c, aarch64-apple-darwin)	8m 1s
build-unix (no-asm-tests)	16m 59s
build-unix (grcov-codecov)	13m 20s
build-unix (bench)	6m 38s
build-unix (doc)	2m 59s
build-unix (cargo-c)	10m 45s
build-unix (check-no-default)	1m 45s
build-unix (check-extra-feats)	3m 58s
build-unix (check-unstable-feats)	3m 41s
build-unix (fuzz)	8m 2s
build-unix (dav1d-tests-arm64, stable)	9m 37s
	3h 9m 12s

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