Only emit EffectsHandler::windowClosed if Deleted got created

Ensures that Effects cannot ref a window during tear down and then
abort because there is no Deleted for the EffectWindow.

effects.cpp

@@ -599,10 +599,12 @@ void EffectsHandlerImpl::slotUnmanagedShown(KWin::Toplevel *t)
     emit windowAdded(u->effectWindow());
 }
 
-void EffectsHandlerImpl::slotWindowClosed(KWin::Toplevel *c)
+void EffectsHandlerImpl::slotWindowClosed(KWin::Toplevel *c, KWin::Deleted *d)
 {
     c->disconnect(this);
-    emit windowClosed(c->effectWindow());
+    if (d) {
+        emit windowClosed(c->effectWindow());
+    }
 }
 
 void EffectsHandlerImpl::slotClientModalityChanged()

effects.h

@@ -245,7 +245,7 @@ protected Q_SLOTS:
     void slotClientShown(KWin::Toplevel*);
     void slotShellClientShown(KWin::Toplevel*);
     void slotUnmanagedShown(KWin::Toplevel*);
-    void slotWindowClosed(KWin::Toplevel *c);
+    void slotWindowClosed(KWin::Toplevel *c, KWin::Deleted *d);
     void slotClientMaximized(KWin::AbstractClient *c, MaximizeMode maxMode);
     void slotOpacityChanged(KWin::Toplevel *t, qreal oldOpacity);
     void slotClientModalityChanged();