Fix use after free of task in FuturesUnordered when dropped future panics #2837

Job	Run time
cargo bench	51s
cargo clippy	26s
cargo +1.36 build (futures-{core, io, sink})	31s
cargo +stable build	32s
cargo doc	19s
cargo +beta build	37s
cargo hack check --feature-powerset	2m 21s
cargo fmt	3s
cargo +nightly build	35s
cargo minimal-versions build	26s
cargo miri test	12m 39s
cargo build --target thumbv7m-none-eabi	25s
cargo test -Z sanitizer=address	3m 17s
cargo build --target thumbv6m-none-eabi	24s
cargo test -Z sanitizer=memory	4m 53s
cargo +1.56 build	1m 0s
cargo test -Z sanitizer=thread	2m 20s
cargo test (ubuntu-latest)	2m 40s
cargo test (macos-latest)	6m 7s
cargo test (windows-latest)	5m 5s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 35s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 37s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 41s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 31s
	1h 5m 55s

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