bigint.c (ucmp): fixed error with zero length zero value.

Big integers have two kind of zero values:

- digits in `p` array are all zeros
- length `sn` is zero

The old code crashed with the latter zero.

mrbgems/mruby-bigint/core/bigint.c

@@ -186,6 +186,7 @@ ucmp(mpz_t *y, mpz_t *x)
 {
   if (y->sz < x->sz) return -1;
   if (y->sz > x->sz) return 1;
+  if (x->sz == 0) return 0;
   for (size_t i=x->sz-1;; i--) {
     mp_limb a = y->p[i];
     mp_limb b = x->p[i];