Add test for Montgomery ladder with a scalar with high bit set.

src/montgomery.rs

@@ -545,6 +545,19 @@ mod test {
 
         assert_eq!(result.compress(), expected.to_montgomery().compress());
     }
+
+    #[test]
+    #[should_panic(expected = "assertion failed: self[31] <= 127")]
+    fn ladder_matches_scalarmult_with_scalar_high_bit_set() {
+        let mut s: Scalar = Scalar::one();
+
+        s[31] = 255;
+
+        let result: MontgomeryPoint = &BASE_COMPRESSED_MONTGOMERY.decompress() * &s;
+        let expected: ExtendedPoint = &constants::ED25519_BASEPOINT_TABLE * &s;
+
+        assert_eq!(result.compress(), expected.to_montgomery().compress())
+    }
 }
 
 #[cfg(all(test, feature = "bench"))]