Fix Sync impl of FuturesUnordered #2331

Job	Run time
cargo clippy	31s
cargo +stable build	55s
codegen	40s
cargo bench	1m 14s
cargo +beta build	1m 17s
cargo doc	35s
cargo +nightly build	53s
cargo +1.36 build (futures-{core, io, sink})	4m 1s
cargo hack check --feature-powerset	5m 2s
cargo fmt	7s
cargo minimal-versions build	46s
cargo miri test	1m 25s
cargo build --target thumbv7m-none-eabi	42s
cargo build --target thumbv6m-none-eabi	41s
cargo +1.56 build	2m 40s
cargo test -Z sanitizer=address	1m 52s
cargo test (ubuntu-latest)	31s
cargo test -Z sanitizer=memory	1m 38s
cargo test (macos-latest)	1m 12s
cargo test -Z sanitizer=thread	1m 24s
cargo test (windows-latest)	1m 44s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	1m 50s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	1m 50s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	2m 0s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	1m 57s
	37m 27s

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