Prevent layershell from crashing when rearranging when we shouldn't

Here's the backtrace that prompted the MR:
```
0  QVector<KWin::VirtualDesktop*>::count() const (this=0x10) at
/home/apol/devel/kde5/include/QtCore/qvector.h:241
1  KWin::VirtualDesktopManager::count() const (this=0x0) at
/home/apol/devel/frameworks/kwin/src/virtualdesktops.h:687
2  KWin::Workspace::updateClientArea(bool) (this=0x0, force=false) at
/home/apol/devel/frameworks/kwin/src/workspace.cpp:2089
3  0x00007fef12a180b3 in KWin::LayerShellV1Integration::rearrange()
(this=<optimized out>) at
/home/apol/devel/frameworks/kwin/src/layershellv1integration.cpp:208
4  0x00007fef13094806 in QtPrivate::QSlotObjectBase::call(QObject*,
void**) (a=0x7ffcf9674f70, r=0x5569d2981a40, this=0x5569d2981e50) at
../../include/QtCore/../../../../../devel/frameworks/qt5/qtbase/src/corelib/kernel/qobjectdefs_impl.h:398
5  doActivate<false>(QObject*, int, void**) (sender=0x5569d2981dc0,
signal_index=3, argv=argv@entry=0x7ffcf9674f70) at
/home/apol/devel/frameworks/qt5/qtbase/src/corelib/kernel/qobject.cpp:3886
6  0x00007fef1308db60 in QMetaObject::activate(QObject*, QMetaObject
const*, int, void**) (sender=<optimized out>, m=m@entry=0x7fef1332d280
<QTimer::staticMetaObject>,
local_signal_index=local_signal_index@entry=0,
argv=argv@entry=0x7ffcf9674f70) at
/home/apol/devel/frameworks/qt5/qtbase/src/corelib/kernel/qobject.cpp:3946
7  0x00007fef1309871a in QTimer::timeout(QTimer::QPrivateSignal)
(this=<optimized out>, _t1=...) at .moc/moc_qtimer.cpp:205
```