Accidentally "SIGSEGV: Illegal storage access" error after arc optimizations (#13325)

[khchen]

Recently, I am trying to update wNim to compile with --gc:arc.
I am almost done. Everything is fine except some bugs (already fixed, thanks to Araq).
I created a branch for my current effort: https://github.com/khchen/wNim/tree/arc

However, the things go bad after following commit:

commit fb641483f0e2ed974b89d629ea5ec28e5e6145ce
Author: Andreas Rumpf <rumpf_a@web.de>
Date:   Wed Mar 18 16:57:34 2020 +0100

    arc optimizations (#13325)

    * scope based destructors
    * handle 'or' and 'and' expressions properly, see the new test arc/tcontrolflow.nim
    * make this branch mergable, logic is disabled for now

The worst thing is that I cannot find out why the error happens. Sometimes it is nonsense for me. For example, the following two code cause different error message:

import winim/winstr
import wNim/[wApp, wFrame, wPanel, wButton]

let frame = Frame()
let panel = Panel(frame)
let button = Button(panel)

import wNim/[wApp, wFrame, wPanel, wButton]

let frame = Frame()
let panel = Panel(frame)
let button = Button(panel)

Sorry for that I failed to simplify the problem, but I am sure both codes works before "arc optimizations (#13325)" commit. Maybe someone can help me.