dd: don't error when outfile is /dev/null

Prevent `dd` from terminating with an error when given the
command-line argument `of=/dev/null`. This commit allows the call to
`File::set_len()` to result in an error without causing the process to
terminate prematurely.

src/uu/dd/src/dd.rs

@@ -521,10 +521,17 @@ impl OutputTrait for Output<File> {
             let mut dst = open_dst(Path::new(&fname), &cflags, &oflags)
                 .map_err_context(|| format!("failed to open {}", fname.quote()))?;
 
+            // Seek to the index in the output file, truncating if requested.
+            //
+            // Calling `set_len()` may result in an error (for
+            // example, when calling it on `/dev/null`), but we don't
+            // want to terminate the process when that happens.
+            // Instead, we suppress the error by calling
+            // `Result::ok()`. This matches the behavior of GNU `dd`
+            // when given the command-line argument `of=/dev/null`.
             let i = seek.unwrap_or(0).try_into().unwrap();
             if !cflags.notrunc {
-                dst.set_len(i)
-                    .map_err_context(|| "failed to truncate output file".to_string())?;
+                dst.set_len(i).ok();
             }
             dst.seek(io::SeekFrom::Start(i))
                 .map_err_context(|| "failed to seek in output file".to_string())?;

tests/by-util/test_dd.rs

@@ -1095,3 +1095,10 @@ fn test_truncated_record() {
         .stdout_is("ac")
         .stderr_is("0+1 records in\n0+1 records out\n2 truncated records\n");
 }
+
+/// Test that the output file can be `/dev/null`.
+#[cfg(unix)]
+#[test]
+fn test_outfile_dev_null() {
+    new_ucmd!().arg("of=/dev/null").succeeds().no_stdout();
+}