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.

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();
+}