Fix use after free of task in FuturesUnordered when dropped future panics #2840

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	31s
cargo +1.56 build	1m 5s
cargo hack check --feature-powerset	2m 23s
cargo clippy	24s
cargo bench	48s
cargo +stable build	34s
cargo +beta build	41s
cargo minimal-versions build	28s
cargo miri test	12m 31s
cargo build --target thumbv7m-none-eabi	27s
cargo +nightly build	33s
cargo build --target thumbv6m-none-eabi	25s
cargo test -Z sanitizer=address	2m 46s
cargo test (ubuntu-latest)	2m 54s
cargo test -Z sanitizer=memory	4m 55s
cargo doc	20s
cargo test (macos-latest)	6m 41s
cargo test -Z sanitizer=thread	2m 29s
cargo test (windows-latest)	4m 53s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 32s
cargo fmt	4s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 30s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 38s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 35s
	1h 6m 7s

Provide feedback