Fix use after free of task in FuturesUnordered when dropped future panics #2841

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	33s
cargo clippy	26s
cargo bench	44s
cargo +stable build	41s
cargo +beta build	39s
cargo doc	17s
cargo hack check --feature-powerset	2m 23s
cargo +nightly build	37s
cargo fmt	5s
cargo minimal-versions build	24s
cargo miri test	12m 46s
cargo build --target thumbv7m-none-eabi	27s
cargo test -Z sanitizer=address	3m 24s
cargo build --target thumbv6m-none-eabi	28s
cargo test -Z sanitizer=memory	4m 54s
cargo test (ubuntu-latest)	2m 52s
cargo +1.56 build	1m 5s
cargo test -Z sanitizer=thread	2m 26s
cargo test (macos-latest)	6m 2s
cargo test (windows-latest)	4m 59s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 23s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 43s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 35s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 28s
	1h 6m 21s

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