py/objint_mpz: Fix pow3 where third argument is zero.

This finding is based on fuzzing MicroPython.  I manually minimized the
test case it provided.

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/objint_mpz.c

@@ -356,9 +356,10 @@ static mpz_t *mp_mpz_for_int(mp_obj_t arg, mpz_t *temp) {
 mp_obj_t mp_obj_int_pow3(mp_obj_t base, mp_obj_t exponent,  mp_obj_t modulus) {
     if (!mp_obj_is_int(base) || !mp_obj_is_int(exponent) || !mp_obj_is_int(modulus)) {
         mp_raise_TypeError(MP_ERROR_TEXT("pow() with 3 arguments requires integers"));
+    } else if (modulus == MP_OBJ_NEW_SMALL_INT(0)) {
+        mp_raise_ValueError(MP_ERROR_TEXT("divide by zero"));
     } else {
-        mp_obj_t result = mp_obj_new_int_from_ull(0); // Use the _from_ull version as this forces an mpz int
-        mp_obj_int_t *res_p = (mp_obj_int_t *)MP_OBJ_TO_PTR(result);
+        mp_obj_int_t *res_p = mp_obj_int_new_mpz();
 
         mpz_t l_temp, r_temp, m_temp;
         mpz_t *lhs = mp_mpz_for_int(base,     &l_temp);
@@ -376,7 +377,7 @@ mp_obj_t mp_obj_int_pow3(mp_obj_t base, mp_obj_t exponent,  mp_obj_t modulus) {
         if (mod == &m_temp) {
             mpz_deinit(mod);
         }
-        return result;
+        return MP_OBJ_FROM_PTR(res_p);
     }
 }
 #endif

tests/basics/builtin_pow3_intbig.py

@@ -20,3 +20,8 @@ print(hex(pow(2, x-1, x))) # Should be 1, since x is prime
 print(hex(pow(y, x-1, x))) # Should be 1, since x is prime
 print(hex(pow(y, y-1, x))) # Should be a 'big value'
 print(hex(pow(y, y-1, y))) # Should be a 'big value'
+
+try:
+    print(pow(1, 2, 0))
+except ValueError:
+    print("ValueError")