fix for b == 0

Author: Christoph Zurnieden

demo/test.c

@@ -1931,6 +1931,15 @@ static int test_mp_root_n(void)
       }
    }
 
+   /* 0^(1/x) = 0 with x != 0 is allowed, test */
+   mp_set(&a, 0);
+   DO(mp_root_n(&a, 2, &c));
+   EXPECT(mp_cmp_d(&c, 0) == MP_EQ);
+
+   /* Not allowed: division by zero */
+   mp_set(&a, 2);
+   EXPECT(mp_root_n(&a, 0, &c) == MP_VAL);
+
    /* root^base == input with small input and base */
    mp_set(&a, 4);
    DO(mp_root_n(&a, 2, &c));

mp_root_n.c

@@ -147,6 +147,17 @@ mp_err mp_root_n(const mp_int *a, int b, mp_int *c)
    int    ilog2;
    mp_err err;
 
+   if (b == 0) {
+      mp_set(c, 0);
+      return MP_VAL;
+   }
+
+   /* 0^(1/x) = 0 with x != 0 is allowed */
+   if (mp_iszero(a)) {
+      mp_set(c, 0);
+      return MP_OKAY;
+   }
+
    if (b < 0 || (unsigned)b > (unsigned)MP_DIGIT_MAX) {
       return MP_VAL;
    }