Fix crash when pinging a surface with an invalid resource.

Check the surface hasn't been deleted before initiating the ping so
that an invalid resource isn't dereferenced when printing a warning
after the operation inevitably fails.

Change-Id: I18c82de280d5a7a5339e3179c2bba49800626bb6
Reviewed-by: Johan Helsing <johan.helsing@theqtcompany.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>

src/compositor/compositor_api/qwaylandsurface.cpp

@@ -361,6 +361,10 @@ bool QWaylandSurface::sendInterfaceOp(QWaylandSurfaceOp &op)
 void QWaylandSurface::ping()
 {
     Q_D(QWaylandSurface);
+
+    if (d->isDestroyed())
+        return;
+
     uint32_t serial = wl_display_next_serial(compositor()->waylandDisplay());
     QWaylandSurfacePingOp op(serial);
     if (!sendInterfaceOp(op)) {