Fix exit crash when running samegame-desktop in qmlscene.

Change-Id: Ib4c162a909eb45e99a7e5c31737587720e9e2deb
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>

src/quick/particles/qquickparticleemitter_p.h

@@ -178,7 +178,8 @@ public slots:
     {
         if (m_system != arg) {
             m_system = arg;
-            m_system->registerParticleEmitter(this);
+            if (m_system)
+                m_system->registerParticleEmitter(this);
             emit systemChanged(arg);
         }
     }