FillBuf: Do not call poll_fill_buf twice #2416

Job	Run time
cargo minimal-versions build	27s
cargo build --target thumbv7m-none-eabi	29s
cargo +stable build	45s
cargo build --target thumbv6m-none-eabi	27s
cargo +1.36 build (futures-{core, io, sink})	33s
cargo test (ubuntu-latest)	3m 21s
cargo +beta build	40s
cargo test (macos-latest)	6m 41s
cargo +nightly build	43s
cargo +1.56 build	1m 0s
cargo test (windows-latest)	4m 59s
cargo bench	50s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 47s
cargo clippy	25s
codegen	27s
cargo hack check --feature-powerset	2m 40s
cargo fmt	12s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 36s
cargo miri test	9m 23s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 10s
cargo test -Z sanitizer=address	5m 40s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 46s
cargo test -Z sanitizer=memory	4m 10s
cargo test -Z sanitizer=thread	2m 47s
	1h 7m 58s

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