sleep: use Duration::saturating_add to sum times

Use `Duration::saturating_add()` to avoid a panic due to overflow when
adding the durations provided as command-line arguments.

src/uu/sleep/src/sleep.rs

@@ -63,7 +63,7 @@ fn sleep(args: &[&str]) -> UResult<()> {
         args.iter().try_fold(
             Duration::new(0, 0),
             |result, arg| match uucore::parse_time::from_str(&arg[..]) {
-                Ok(m) => Ok(m + result),
+                Ok(m) => Ok(m.saturating_add(result)),
                 Err(f) => Err(USimpleError::new(1, f)),
             },
         )?;

tests/by-util/test_sleep.rs

@@ -131,3 +131,13 @@ fn test_dont_overflow() {
         .no_stderr()
         .no_stdout();
 }
+
+// #[test]
+#[allow(dead_code)]
+fn test_sum_overflow() {
+    new_ucmd!()
+        .args(&["100000000000000d", "100000000000000d", "100000000000000d"])
+        .succeeds()
+        .no_stderr()
+        .no_stdout();
+}