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_timeout.rs

@@ -1,3 +1,4 @@
+// spell-checker:ignore dont
 use crate::common::util::*;
 
 // FIXME: this depends on the system having true and false in PATH
@@ -64,3 +65,19 @@ fn test_preserve_status() {
         .no_stderr()
         .no_stdout();
 }
+
+#[test]
+fn test_dont_overflow() {
+    new_ucmd!()
+        .args(&["9223372036854775808d", "sleep", "0"])
+        .succeeds()
+        .code_is(0)
+        .no_stderr()
+        .no_stdout();
+    new_ucmd!()
+        .args(&["-k", "9223372036854775808d", "10", "sleep", "0"])
+        .succeeds()
+        .code_is(0)
+        .no_stderr()
+        .no_stdout();
+}