Fixed unsoundness in `StderrForwarder.forward_available` #1832

Job	Run time
Test CUDA support	27s
Rustfmt	12s
Clippy	13s
Test build-std (aarch64-tvos)	22s
Test build-std (aarch64-tvos-sim)	22s
semver-checks	2m 28s
Test build-std (x86_64-tvos)	23s
Test (stable)	23s
Test wasm (wasm32-unknown-unknown)	7s
Test (beta)	18s
MSRV (ubuntu-latest)	26s
Test (nightly)	19s
MSRV (windows-latest)	38s
Test (linux32)	29s
MSRV (macos-latest)	37s
Test (macos)	22s
Test (aarch64-macos)	20s
Test (aarch64-ios)	23s
Test (win32)	20s
Test (win64)	26s
Test (mingw32)	27s
Test (mingw64)	22s
Test (windows-2019)	28s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	28s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	28s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	28s
	12m 16s

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