Fix issues with `AsyncBufRead::read_line` and `AsyncBufReadExt::lines` #2816

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	1m 43s
cargo test (ubuntu-latest)	2m 39s
cargo test (macos-latest)	7m 3s
cargo test (windows-latest)	4m 55s
cargo hack check --feature-powerset	2m 22s
cargo bench	47s
cargo +stable build	38s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 42s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 25s
cargo miri test	3m 34s
cargo +beta build	41s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 45s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 26s
cargo +nightly build	34s
cargo build --target thumbv7m-none-eabi	29s
cargo doc	20s
cargo build --target thumbv6m-none-eabi	27s
cargo clippy	24s
cargo fmt	13s
cargo minimal-versions build	26s
cargo test -Z sanitizer=address	3m 2s
cargo test -Z sanitizer=memory	3m 39s
cargo +1.56 build	2m 12s
cargo test -Z sanitizer=thread	2m 27s
	58m 53s

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