Add a test for complex divide by zero.

src/libnum/complex.rs

@@ -257,6 +257,14 @@ mod test {
         assert_eq!(_1_0i.inv(), _1_0i.inv());
     }
 
+    #[test]
+    #[should_fail]
+    fn test_divide_by_zero_natural() {
+        let n = Complex::new(2i, 3i);
+        let d = Complex::new(0, 0);
+        let _x = n / d;
+    }
+
     #[test]
     #[should_fail]
     #[ignore]