Fix compile error by #3309

src/array.c

@@ -244,10 +244,12 @@ mrb_ary_s_create(mrb_state *mrb, mrb_value self)
 static void
 ary_concat(mrb_state *mrb, struct RArray *a, struct RArray *a2)
 {
+  mrb_int len;
+
   if (a2->len > ARY_MAX_SIZE - a->len) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "array size too big");
   }
-  mrb_int len = a->len + a2->len;
+  len = a->len + a2->len;
 
   ary_modify(mrb, a);
   if (a->aux.capa < len) {