mruby-bigint (uzero_p): check from the top, not bottom

For most of the case, the first non-zero value would be found earlier
when search starts from the top.

mrbgems/mruby-bigint/core/bigint.c

@@ -249,7 +249,7 @@ static int
 uzero_p(mpz_t *x)
 {
   if (x->sz == 0) return 1;
-  for (size_t i=0; i < x->sz; i++)
+  for (size_t i=x->sz-1; 0 <= i; i--)
     if (x->p[i] != 0)
       return 0;
   return 1;