The chunk-size-zero regression test spawned fcry with stdin piped and
unconditionally unwrapped the write into that pipe. That made the test depend
on scheduler timing: the child may validate --chunk-size 0, report the error,
and exit before it has drained stdin.
Treat BrokenPipe as the expected early-exit shape for this failing command,
while still failing on any other write error and still asserting that the
process exits unsuccessfully. The valid empty-stdin chunk-size case remains
unchanged.
Test Plan:
- cargo fmt --check
- cargo test
- cargo clippy --all-targets -- -D warnings
- git diff --check
Refs: none
ddidderr
3f53c221c8
