uucore: use Duration::saturating_mul in parse_time

Use `Duration::saturating_mul()` to avoid a panic due to overflow in
`uucore::parse_time::from_str()`. This change prevents panic on very
large arguments to timeout and sleep.

tests/by-util/test_sleep.rs

@@ -1,3 +1,4 @@
+// spell-checker:ignore dont
 use crate::common::util::*;
 
 use std::time::{Duration, Instant};
@@ -115,3 +116,18 @@ fn test_sleep_sum_duration_many() {
 fn test_sleep_wrong_time() {
     new_ucmd!().args(&["0.1s", "abc"]).fails();
 }
+
+// TODO These tests would obviously block for a very long time. We
+// only want to verify that there is no error here, so we could just
+// figure out a way to terminate the child process after a short
+// period of time.
+
+// #[test]
+#[allow(dead_code)]
+fn test_dont_overflow() {
+    new_ucmd!()
+        .arg("9223372036854775808d")
+        .succeeds()
+        .no_stderr()
+        .no_stdout();
+}