Cleanup RemoteAccess buffers on aboutToBeUnbound instead of object destruction

Summary:
The destructor or Resource::~Resource will delete the d-pointer used by
Resource before we reach QObject::~QObject() of the resource which emits
the destroyed signal. This means this lamba is accessing deleted
contents.

Based on
https://build.kde.org/view/Frameworks/job/Frameworks%20kwayland%20kf5-qt5%20SUSEQt5.10/71/testReport/

aboutToBeUnbound is emitted before object destruction.

Test Plan: Relevant unit test still passes and this lambda is still invoked.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D14291

src/wayland/server/remote_access_interface.cpp

@@ -241,13 +241,12 @@ void RemoteAccessManagerInterface::Private::getBufferCallback(wl_client *client,
         return;
     }
 
-    QObject::connect(rbuf, &QObject::destroyed, [p, rbuf, resource, &bh] {
+    QObject::connect(rbuf, &Resource::aboutToBeUnbound, p->q, [p, rbuf, resource, &bh] {
         if (!p->clientResources.contains(resource)) {
             // remote buffer destroy confirmed after client is already gone
             // all relevant buffers are already unreferenced
             return;
         }
-
         qCDebug(KWAYLAND_SERVER) << "Remote buffer returned, client" << wl_resource_get_id(resource)
                                                      << ", id" << rbuf->id()
                                                      << ", fd" << bh.buf->fd();