BUG: sanitizer found use-after-free in WOutputLayoutItem's updateOutputs

- precond: 
  - WLR_X11_OUTPUTS=2
  - -fsanitizer=address
- run
  - tinywl-qtquick
  - `qtcreator -platform wayland`

Result:
```
=> updateOutputs()
  => ~QList<QPointer<WOutput>>()
=> updateOutputs
  => QPointer<WOutput>() **use-after-free**
```
if use manually deep-copy or std::vector, won't trigger