test: tolerate closed stdin in chunk-size failure

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

tests/roundtrip.rs

@@ -7,7 +7,7 @@
 // wrong key, truncation, bad magic).
 
 use std::fs;
-use std::io::Write;
+use std::io::{ErrorKind, Write};
 use std::process::{Command, Stdio};
 
 use assert_cmd::cargo::CommandCargoExt;
@@ -915,7 +915,14 @@ fn stdin_chunk_size_zero_fails_but_empty_valid_chunk_succeeds() {
         .stderr(Stdio::piped())
         .spawn()
         .unwrap();
-    bad.stdin.as_mut().unwrap().write_all(b"x").unwrap();
+    // Invalid options can make the child exit before it drains stdin.
+    if let Err(err) = bad.stdin.as_mut().unwrap().write_all(b"x") {
+        assert_eq!(
+            err.kind(),
+            ErrorKind::BrokenPipe,
+            "unexpected stdin write error for failing chunk-size 0 process: {err}"
+        );
+    }
     let bad_out = bad.wait_with_output().unwrap();
     assert!(!bad_out.status.success(), "chunk-size 0 should fail");