Add more checks before accessing struct pointer; ref #3831

mrbgems/mruby-struct/src/struct.c

@@ -147,11 +147,13 @@ static mrb_value
 mrb_struct_set_m(mrb_state *mrb, mrb_value obj)
 {
   mrb_int i = mrb_fixnum(mrb_proc_cfunc_env_get(mrb, 0));
-  mrb_value *ptr = RSTRUCT_PTR(obj);
+  mrb_value *ptr;
   mrb_value val;
+
   mrb_get_args(mrb, "o", &val);
   mrb_struct_modify(mrb, obj);
-  if (i >= RSTRUCT_LEN(obj)) {
+  ptr = RSTRUCT_PTR(obj);
+  if (ptr == NULL || i >= RSTRUCT_LEN(obj)) {
     mrb_ary_set(mrb, obj, i, val);
   }
   else {