fix overflow sum (#900)

Co-authored-by: Mauro Toscano <12560266+MauroToscano@users.noreply.github.com>

math/src/field/fields/montgomery_backed_prime_fields.rs

@@ -224,15 +224,17 @@ where
                 if v <= u {
                     u = u - v;
                     if b < c {
-                        b = b + modulus;
+                        b = modulus - c + b;
+                    } else {
+                        b = b - c;
                     }
-                    b = b - c;
                 } else {
                     v = v - u;
                     if c < b {
-                        c = c + modulus;
+                        c = modulus - b + c;
+                    } else {
+                        c = c - b;
                     }
-                    c = c - b;
                 }
             }
 
@@ -1243,6 +1245,14 @@ mod tests_u256_prime_fields {
         assert_eq!(minus_3_pow_2, nine);
     }
 
+    #[test]
+    fn secp256k1_inv_works() {
+        let a = SecpMontElement::from_hex_unchecked("0x456");
+        let a_inv = a.inv().unwrap();
+
+        assert_eq!(a * a_inv, SecpMontElement::one());
+    }
+
     #[test]
     fn test_cios_overflow_case() {
         let a = GoldilocksElement::from(732582227915286439);