Check whether Cursor is valid before using from EffectsHandler mousePolling

During tear down Compositor and Effect system might be destroyed after
Cursor. Thus we need to ensure that we don't call into Cursor
unconditonally.

effects.cpp

@@ -769,12 +769,14 @@ void* EffectsHandlerImpl::getProxy(QString name)
 
 void EffectsHandlerImpl::startMousePolling()
 {
-    Cursor::self()->startMousePolling();
+    if (Cursor::self())
+        Cursor::self()->startMousePolling();
 }
 
 void EffectsHandlerImpl::stopMousePolling()
 {
-    Cursor::self()->stopMousePolling();
+    if (Cursor::self())
+        Cursor::self()->stopMousePolling();
 }
 
 bool EffectsHandlerImpl::hasKeyboardGrab() const