Fix issues with `AsyncBufRead::read_line` and `AsyncBufReadExt::lines… #2823

Job	Run time
cargo clippy	25s
cargo bench	47s
cargo +stable build	34s
cargo +1.36 build (futures-{core, io, sink})	27s
cargo +beta build	37s
cargo doc	18s
cargo hack check --feature-powerset	2m 25s
cargo fmt	3s
cargo +nightly build	34s
cargo minimal-versions build	25s
cargo miri test	12m 33s
cargo build --target thumbv7m-none-eabi	27s
cargo test -Z sanitizer=address	3m 23s
cargo build --target thumbv6m-none-eabi	27s
cargo test (ubuntu-latest)	2m 51s
cargo +1.56 build	1m 0s
cargo test -Z sanitizer=memory	3m 54s
cargo test (macos-latest)	6m 13s
cargo test -Z sanitizer=thread	2m 21s
cargo test (windows-latest)	4m 37s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 49s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 49s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 16s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 40s
	1h 5m 55s

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