Fix panic safety issue in StderrForwarder #1547

Job	Run time
Test (stable)	21s
Test (beta)	28s
Test (nightly)	25s
Clippy	11s
Test build-std (aarch64-tvos)	3m 15s
Test (linux32)	1m 9s
Test CUDA support	36s
Test build-std (aarch64-tvos-sim)	1m 24s
Test (macos)	2m 22s
Test build-std (x86_64-tvos)	1m 28s
MSRV (ubuntu-latest)	20s
Test (aarch64-macos)	41s
Rustfmt	9s
MSRV (windows-latest)	37s
Test (aarch64-ios)	1m 1s
Test (win32)	1m 9s
Test (win64)	1m 21s
semver-checks	2m 19s
Test (mingw32)	1m 21s
Test (mingw64)	1m 55s
Test (windows-2019)	1m 59s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 12s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	2m 19s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	2m 6s
	32m 8s

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