factor::miller_rabin::is_prime: Fix bug

Montgomery only works for odd n, so attempting to construct an instance
for an even number results in a panic!

The most obvious solution is to special-case even numbers.

src/uu/factor/src/miller_rabin.rs

@@ -84,11 +84,15 @@ pub(crate) fn test<A: Arithmetic>(m: A) -> Result {
     Prime
 }
 
-// Used by build.rs' tests
+// Used by build.rs' tests and debug assertions
 #[allow(dead_code)]
 pub(crate) fn is_prime(n: u64) -> bool {
+    if n % 2 == 0 {
+        n == 2
+    } else {
         test::<Montgomery>(Montgomery::new(n)).is_prime()
     }
+}
 
 #[cfg(test)]
 mod tests {