[server] Add resource check in QtExtendedSurfaceInterface::close

Summary:
The QtExtendedSurfaceInterface might be unbound and thus the resource
might be null when calling into close. Thus we need to do a nullptr
check.

Hit a crash there.

Test Plan:
Unfortunately no test case as we don't have a client side
implementation for this.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D1961

src/wayland/server/qtsurfaceextension_interface.cpp

@@ -219,6 +219,9 @@ QtExtendedSurfaceInterface::Private *QtExtendedSurfaceInterface::d_func() const
 void QtExtendedSurfaceInterface::close()
 {
     Q_D();
+    if (!d->resource) {
+        return;
+    }
     qt_extended_surface_send_close(d->resource);
     d->client->flush();
 }